Starting plasma-kwin_x11.service times out with no clue in debug log

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Yesterday on Fedora 42 system I did an update with sudo dnf update -y and today when connecting remotely to this system using TigerVNC I see Plasma splash but then there’s no session, only black screen with cursor.
As part of this last system update I see kwin-x11 package was upgraded from kwin-x11-0:6.3.4 to kwin-x11-0:6.3.5 and tigervnc was not upgraded so I suspect that kwin-x11 6.3.5 introduced some bug.
Below is output from journalctl --user showing kwin-x11 trying to start a couple of times and then dumping core (with some part of core dump omitted due to 32000 characters limit for post here).

$ kinfo
Operating System: Fedora Linux 42
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0
Kernel Version: 6.14.5-300.fc42.x86_64 (64-bit)
Graphics Platform: offscreen
Processors: 12 × AMD Ryzen 5 7600 6-Core Processor
Memory: 14.8 GiB of RAM
Output from `journalctl --user` 
maj 13 10:37:10 demon systemd[1330]: Reload requested from client PID 1089381 ('startplasma-x11')...
maj 13 10:37:10 demon systemd[1330]: Reloading...
maj 13 10:37:11 demon systemd[1330]: Reloading finished in 98 ms.
maj 13 10:37:11 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:37:21 demon sudo[1089358]: pam_unix(sudo:session): session closed for user root
maj 13 10:37:33 demon kwin_x11[1089899]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:37:34 demon kwin_x11[1089899]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:37:56 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:37:56 demon kwin_x11[1089899]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:37:56 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:37:56 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:37:56 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:37:56 demon systemd[1330]: plasma-kwin_x11.service: Consumed 585ms CPU time, 92.2M memory peak.
maj 13 10:37:56 demon systemd[1330]: Reached target plasma-workspace-x11.target.
maj 13 10:37:56 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 1.
maj 13 10:37:56 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:37:56 demon systemd[1330]: Started dbus-:1.2-org.kde.KSplash@1.service.
maj 13 10:38:41 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:38:41 demon kwin_x11[1093574]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:38:41 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:38:41 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:38:41 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:38:41 demon systemd[1330]: plasma-kwin_x11.service: Consumed 370ms CPU time, 79M memory peak.
maj 13 10:38:41 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 2.
maj 13 10:38:42 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:38:56 demon plasma_waitforname[1093630]: org.kde.plasma.waitforname: WaitForName: Service was not registered within timeout
maj 13 10:38:56 demon systemd[1330]: dbus-:1.2-org.kde.KSplash@1.service: Main process exited, code=exited, status=1/FAILURE
maj 13 10:38:56 demon systemd[1330]: dbus-:1.2-org.kde.KSplash@1.service: Failed with result 'exit-code'.
maj 13 10:39:27 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:39:27 demon kwin_x11[1097314]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:39:27 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:39:27 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:39:27 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:39:27 demon systemd[1330]: plasma-kwin_x11.service: Consumed 352ms CPU time, 78.6M memory peak.
maj 13 10:39:27 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 3.
maj 13 10:39:27 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:39:27 demon systemd[1330]: Started dbus-:1.2-org.kde.KSplash@2.service.
maj 13 10:40:12 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:40:12 demon kwin_x11[1101017]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:40:12 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:40:12 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:40:12 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:40:12 demon systemd[1330]: plasma-kwin_x11.service: Consumed 348ms CPU time, 78.7M memory peak.
maj 13 10:40:12 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 4.
maj 13 10:40:13 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:40:27 demon plasma_waitforname[1101038]: org.kde.plasma.waitforname: WaitForName: Service was not registered within timeout
maj 13 10:40:27 demon systemd[1330]: dbus-:1.2-org.kde.KSplash@2.service: Main process exited, code=exited, status=1/FAILURE
maj 13 10:40:27 demon systemd[1330]: dbus-:1.2-org.kde.KSplash@2.service: Failed with result 'exit-code'.
maj 13 10:40:40 demon kwin_x11[1104725]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:40:48 demon kwin_x11[1104725]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:40:58 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:40:58 demon kwin_x11[1104725]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:40:58 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:40:58 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:40:58 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:40:58 demon systemd[1330]: plasma-kwin_x11.service: Consumed 389ms CPU time, 87.4M memory peak.
maj 13 10:40:58 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 5.
maj 13 10:40:58 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:40:58 demon systemd[1330]: Started dbus-:1.2-org.kde.KSplash@3.service.
maj 13 10:41:43 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:41:43 demon kwin_x11[1108431]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:41:43 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:41:43 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:41:43 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:41:43 demon systemd[1330]: plasma-kwin_x11.service: Consumed 384ms CPU time, 78.6M memory peak.
maj 13 10:41:43 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 6.
maj 13 10:41:44 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:41:58 demon plasma_waitforname[1108452]: org.kde.plasma.waitforname: WaitForName: Service was not registered within timeout
maj 13 10:41:58 demon systemd[1330]: dbus-:1.2-org.kde.KSplash@3.service: Main process exited, code=exited, status=1/FAILURE
maj 13 10:41:58 demon systemd[1330]: dbus-:1.2-org.kde.KSplash@3.service: Failed with result 'exit-code'.
maj 13 10:42:29 demon systemd[1330]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 10:42:29 demon kwin_x11[1112135]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 10:42:29 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 10:42:29 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 10:42:29 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:42:29 demon systemd[1330]: plasma-kwin_x11.service: Consumed 454ms CPU time, 92.2M memory peak.
maj 13 10:42:29 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 7.
maj 13 10:42:29 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:42:29 demon systemd[1330]: Started dbus-:1.2-org.kde.KSplash@4.service.
maj 13 10:43:07 demon sudo[1119004]:    piotr : TTY=pts/1 ; PWD=/home/piotr ; USER=root ; COMMAND=/usr/bin/systemctl stop vncserver@:1.service
maj 13 10:43:07 demon sudo[1119004]: pam_unix(sudo:session): session opened for user root(uid=0) by piotr(uid=1000)
maj 13 10:43:07 demon kwin_x11[1115854]: The X11 connection broke (error 1). Did the X11 server die?
maj 13 10:43:07 demon sudo[1119004]: pam_unix(sudo:session): session closed for user root
maj 13 10:43:07 demon kwin_x11[1115854]: The X11 connection broke: I/O error (code 1)
maj 13 10:43:07 demon kwin_x11[1115854]: XIO:  fatal IO error 25 (Inappropriate ioctl for device) on X server ":1"
maj 13 10:43:07 demon kwin_x11[1115854]:       after 44 requests (22 known processed) with 0 events remaining.
maj 13 10:43:07 demon kwin_x11[1115854]: QThreadStorage: entry 10 destroyed before end of thread 0x559134872710
maj 13 10:43:07 demon kwin_x11[1115854]: QThreadStorage: entry 9 destroyed before end of thread 0x559134872710
maj 13 10:43:07 demon kwin_x11[1115854]: QThreadStorage: entry 8 destroyed before end of thread 0x559134872710
maj 13 10:43:07 demon kwin_x11[1115854]: QThreadStorage: entry 7 destroyed before end of thread 0x559134872710
maj 13 10:43:07 demon kwin_x11[1115854]: QThreadStorage: entry 1 destroyed before end of thread 0x559134872710
maj 13 10:43:07 demon kwin_x11[1115854]: QThreadStorage: entry 0 destroyed before end of thread 0x559134872710
maj 13 10:43:07 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=exited, status=1/FAILURE
maj 13 10:43:07 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'exit-code'.
maj 13 10:43:07 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 10:43:07 demon systemd[1330]: plasma-kwin_x11.service: Consumed 352ms CPU time, 78.6M memory peak.
maj 13 10:43:08 demon systemd[1330]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 8.
maj 13 10:43:08 demon systemd[1330]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 10:43:08 demon kwin_x11[1119019]: qt.qpa.xcb: could not connect to display :1
maj 13 10:43:08 demon kwin_x11[1119019]: qt.qpa.plugin: From 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.
maj 13 10:43:08 demon kwin_x11[1119019]: qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
maj 13 10:43:08 demon kwin_x11[1119019]: This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

                                         Available platform plugins are: wayland-egl, wayland, eglfs, linuxfb, minimal, minimalegl, offscreen, vkkhrdisplay, vnc, xcb.
maj 13 10:43:08 demon systemd-coredump[1119062]: [🡕] Process 1119019 (kwin_x11) of user 1000 dumped core.

                                                 Module /usr/bin/kwin_x11 from rpm kwin-x11-6.3.5-1.fc42.x86_64
                                                 Module libuuid.so.1 from rpm util-linux-2.40.4-7.fc42.x86_64
                                                 Module libICE.so.6 from rpm libICE-1.1.2-2.fc42.x86_64
                                                 Module libSM.so.6 from rpm libSM-1.2.5-2.fc42.x86_64
                                                 Module libX11-xcb.so.1 from rpm libX11-1.8.11-1.fc42.x86_64
                                                 Module libQt6XcbQpa.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libqxcb.so from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libcrypt.so.2 from rpm libxcrypt-4.4.38-7.fc42.x86_64
                                                 Module libsasl2.so.3 from rpm cyrus-sasl-2.1.28-30.fc42.x86_64
                                                 Module libevent-2.1.so.7 from rpm libevent-2.1.12-15.fc42.x86_64
                                                 Module libunistring.so.5 from rpm libunistring-1.1-9.fc42.x86_64
                                                 Module libgmodule-2.0.so.0 from rpm glib2-2.84.1-1.fc42.x86_64
                                                 Module libpsl.so.5 from rpm libpsl-0.21.5-5.fc42.x86_64
                                                 Module libssh.so.4 from rpm libssh-0.11.1-4.fc42.x86_64
                                                 Module libidn2.so.0 from rpm libidn2-2.3.8-1.fc42.x86_64
                                                 Module libnghttp2.so.14 from rpm nghttp2-1.64.0-3.fc42.x86_64
                                                 Module libimobiledevice-glue-1.0.so.0 from rpm libimobiledevice-glue-1.3.1-2.fc42.x86_64
                                                 Module libusbmuxd-2.0.so.7 from rpm libusbmuxd-2.1.0-3.fc42.x86_64
                                                 Module libssl.so.3 from rpm openssl-3.2.4-3.fc42.x86_64
                                                 Module libduktape.so.207 from rpm duktape-2.7.0-9.fc42.x86_64
                                                 Module libgio-2.0.so.0 from rpm glib2-2.84.1-1.fc42.x86_64
                                                 Module libcurl.so.4 from rpm curl-8.11.1-4.fc42.x86_64
                                                 Module libKF6BreezeIcons.so.6 from rpm kf6-breeze-icons-6.13.0-2.fc42.x86_64
                                                 Module libselinux.so.1 from rpm libselinux-3.8-1.fc42.x86_64
                                                 Module libblkid.so.1 from rpm util-linux-2.40.4-7.fc42.x86_64
                                                 Module libattr.so.1 from rpm attr-2.5.2-5.fc42.x86_64
                                                 Module libplist-2.0.so.4 from rpm libplist-2.6.0-2.fc42.x86_64
                                                 Module libimobiledevice-1.0.so.6 from rpm libimobiledevice-1.3.0^20240916gited9703d-5.fc42.x86_64
                                                 Module libpxbackend-1.0.so from rpm libproxy-0.5.8-2.fc42.x86_64
                                                 Module libbrotlicommon.so.1 from rpm brotli-1.1.0-6.fc42.x86_64
                                                 Module libkeyutils.so.1 from rpm keyutils-1.6.3-5.fc42.x86_64
                                                 Module libkrb5support.so.0 from rpm krb5-1.21.3-5.fc42.x86_64
                                                 Module libcom_err.so.2 from rpm e2fsprogs-1.47.2-3.fc42.x86_64
                                                 Module libk5crypto.so.3 from rpm krb5-1.21.3-5.fc42.x86_64
                                                 Module libkrb5.so.3 from rpm krb5-1.21.3-5.fc42.x86_64
                                                 Module libKF6IconThemes.so.6 from rpm kf6-kiconthemes-6.13.0-2.fc42.x86_64
                                                 Module libKF6IconWidgets.so.6 from rpm kf6-kiconthemes-6.13.0-2.fc42.x86_64
                                                 Module libKF6ItemViews.so.6 from rpm kf6-kitemviews-6.13.0-1.fc42.x86_64
                                                 Module libQt6PrintSupport.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libgobject-2.0.so.0 from rpm glib2-2.84.1-1.fc42.x86_64
                                                 Module libgudev-1.0.so.0 from rpm libgudev-238-7.fc42.x86_64
                                                 Module libxcb-util.so.1 from rpm xcb-util-0.4.1-7.fc42.x86_64
                                                 Module libmount.so.1 from rpm util-linux-2.40.4-7.fc42.x86_64
                                                 Module libacl.so.1 from rpm acl-2.3.2-3.fc42.x86_64
                                                 Module libKF6AuthCore.so.6 from rpm kf6-kauth-6.13.0-1.fc42.x86_64
                                                 Module libKF6Solid.so.6 from rpm kf6-solid-6.13.0-1.fc42.x86_64
                                                 Module libwayland-cursor.so.0 from rpm wayland-1.23.1-1.fc42.x86_64
                                                 Module libicudata.so.76 from rpm icu-76.1-4.fc42.x86_64
                                                 Module libgraphite2.so.3 from rpm graphite2-1.3.14-18.fc42.x86_64
                                                 Module libpcre2-8.so.0 from rpm pcre2-10.45-1.fc42.x86_64
                                                 Module libxml2.so.2 from rpm libxml2-2.12.10-1.fc42.x86_64
                                                 Module libGLdispatch.so.0 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libproxy.so.1 from rpm libproxy-0.5.8-2.fc42.x86_64
                                                 Module libbrotlidec.so.1 from rpm brotli-1.1.0-6.fc42.x86_64
                                                 Module libgssapi_krb5.so.2 from rpm krb5-1.21.3-5.fc42.x86_64
                                                 Module libQt6QmlWorkerScript.so.6 from rpm qt6-qtdeclarative-6.9.0-2.fc42.x86_64
                                                 Module libKF6XmlGui.so.6 from rpm kf6-kxmlgui-6.13.0-2.fc42.x86_64
                                                 Module libKF6IdleTime.so.6 from rpm kf6-kidletime-6.13.0-1.fc42.x86_64
                                                 Module libwacom.so.9 from rpm libwacom-2.15.0-1.fc42.x86_64
                                                 Module libevdev.so.2 from rpm libevdev-1.13.4-1.fc42.x86_64
                                                 Module libmtdev.so.1 from rpm mtdev-1.1.6-10.fc42.x86_64
                                                 Module libexpat.so.1 from rpm expat-2.7.1-1.fc42.x86_64
                                                 Module libxcb-image.so.0 from rpm xcb-util-image-0.4.1-7.fc42.x86_64
                                                 Module libxcb-render-util.so.0 from rpm xcb-util-renderutil-0.3.10-7.fc42.x86_64
                                                 Module libKF6KIOCore.so.6 from rpm kf6-kio-6.13.0-2.fc42.x86_64
                                                 Module libKF6JobWidgets.so.6 from rpm kf6-kjobwidgets-6.13.0-2.fc42.x86_64
                                                 Module libKF6KIOGui.so.6 from rpm kf6-kio-6.13.0-2.fc42.x86_64
                                                 Module libcap.so.2 from rpm libcap-2.73-2.fc42.x86_64
                                                 Module libffi.so.8 from rpm libffi-3.4.6-5.fc42.x86_64
                                                 Module libogg.so.0 from rpm libogg-1.3.5-11.fc42.x86_64
                                                 Module libvorbis.so.0 from rpm libvorbis-1.3.7-12.fc42.x86_64
                                                 Module libQt6WaylandClient.so.6 from rpm qt6-qtwayland-6.9.0-3.fc42.x86_64
                                                 Module liblzma.so.5 from rpm xz-5.8.1-2.fc42.x86_64
                                                 Module libbz2.so.1 from rpm bzip2-1.0.8-20.fc42.x86_64
                                                 Module libcrypto.so.3 from rpm openssl-3.2.4-3.fc42.x86_64
                                                 Module libpcre2-16.so.0 from rpm pcre2-10.45-1.fc42.x86_64
                                                 Module libb2.so.1 from rpm libb2-0.98.1-13.fc42.x86_64
                                                 Module libdouble-conversion.so.3 from rpm double-conversion-3.3.1-1.fc42.x86_64
                                                 Module libsystemd.so.0 from rpm systemd-257.5-6.fc42.x86_64
                                                 Module libzstd.so.1 from rpm zstd-1.5.7-1.fc42.x86_64
                                                 Module libicuuc.so.76 from rpm icu-76.1-4.fc42.x86_64
                                                 Module libicui18n.so.76 from rpm icu-76.1-4.fc42.x86_64
                                                 Module libdbus-1.so.3 from rpm dbus-1.16.0-3.fc42.x86_64
                                                 Module libz.so.1 from rpm zlib-ng-2.2.4-3.fc42.x86_64
                                                 Module libfreetype.so.6 from rpm freetype-2.13.3-2.fc42.x86_64
                                                 Module libharfbuzz.so.0 from rpm harfbuzz-10.4.0-1.fc42.x86_64
                                                 Module libpng16.so.16 from rpm libpng-1.6.44-2.fc42.x86_64
                                                 Module libOpenGL.so.0 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libGLX.so.0 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libglib-2.0.so.0 from rpm glib2-2.84.1-1.fc42.x86_64
                                                 Module libfontconfig.so.1 from rpm fontconfig-2.16.0-2.fc42.x86_64
                                                 Module libEGL.so.1 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libKF6Codecs.so.6 from rpm kf6-kcodecs-6.13.0-1.fc42.x86_64
                                                 Module libKF6WidgetsAddons.so.6 from rpm kf6-kwidgetsaddons-6.13.0-2.fc42.x86_64
                                                 Module libXfixes.so.3 from rpm libXfixes-6.0.1-5.fc42.x86_64
                                                 Module libxcb-res.so.0 from rpm libxcb-1.17.0-5.fc42.x86_64
                                                 Module libQt6Xml.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libQt6Network.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libQt6QmlModels.so.6 from rpm qt6-qtdeclarative-6.9.0-2.fc42.x86_64
                                                 Module libQt6QmlMeta.so.6 from rpm qt6-qtdeclarative-6.9.0-2.fc42.x86_64
                                                 Module libKF6Notifications.so.6 from rpm kf6-knotifications-6.13.0-2.fc42.x86_64
                                                 Module libKF6Package.so.6 from rpm kf6-kpackage-6.13.0-1.fc42.x86_64
                                                 Module libxcvt.so.0 from rpm libxcvt-0.1.2-9.fc42.x86_64
                                                 Module libdisplay-info.so.2 from rpm libdisplay-info-0.2.0-3.fc42.x86_64
                                                 Module libQt6OpenGL.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libb2.so.1 from rpm libb2-0.98.1-13.fc42.x86_64
                                                 Module libdouble-conversion.so.3 from rpm double-conversion-3.3.1-1.fc42.x86_64
                                                 Module libsystemd.so.0 from rpm systemd-257.5-6.fc42.x86_64
                                                 Module libzstd.so.1 from rpm zstd-1.5.7-1.fc42.x86_64
                                                 Module libicuuc.so.76 from rpm icu-76.1-4.fc42.x86_64
                                                 Module libicui18n.so.76 from rpm icu-76.1-4.fc42.x86_64
                                                 Module libdbus-1.so.3 from rpm dbus-1.16.0-3.fc42.x86_64
                                                 Module libz.so.1 from rpm zlib-ng-2.2.4-3.fc42.x86_64
                                                 Module libfreetype.so.6 from rpm freetype-2.13.3-2.fc42.x86_64
                                                 Module libharfbuzz.so.0 from rpm harfbuzz-10.4.0-1.fc42.x86_64
                                                 Module libpng16.so.16 from rpm libpng-1.6.44-2.fc42.x86_64
                                                 Module libOpenGL.so.0 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libGLX.so.0 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libglib-2.0.so.0 from rpm glib2-2.84.1-1.fc42.x86_64
                                                 Module libfontconfig.so.1 from rpm fontconfig-2.16.0-2.fc42.x86_64
                                                 Module libEGL.so.1 from rpm libglvnd-1.7.0-7.fc42.x86_64
                                                 Module libKF6Codecs.so.6 from rpm kf6-kcodecs-6.13.0-1.fc42.x86_64
                                                 Module libKF6WidgetsAddons.so.6 from rpm kf6-kwidgetsaddons-6.13.0-2.fc42.x86_64
                                                 Module libXfixes.so.3 from rpm libXfixes-6.0.1-5.fc42.x86_64
                                                 Module libxcb-res.so.0 from rpm libxcb-1.17.0-5.fc42.x86_64
                                                 Module libQt6Xml.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libQt6Network.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libQt6QmlModels.so.6 from rpm qt6-qtdeclarative-6.9.0-2.fc42.x86_64
                                                 Module libQt6QmlMeta.so.6 from rpm qt6-qtdeclarative-6.9.0-2.fc42.x86_64
                                                 Module libKF6Notifications.so.6 from rpm kf6-knotifications-6.13.0-2.fc42.x86_64
                                                 Module libKF6Package.so.6 from rpm kf6-kpackage-6.13.0-1.fc42.x86_64
                                                 Module libxcvt.so.0 from rpm libxcvt-0.1.2-9.fc42.x86_64
                                                 Module libdisplay-info.so.2 from rpm libdisplay-info-0.2.0-3.fc42.x86_64
                                                 Module libQt6OpenGL.so.6 from rpm qt6-qtbase-6.9.0-2.fc42.x86_64
                                                 Module libKScreenLocker.so.6 from rpm kscreenlocker-6.3.5-1.fc42.x86_64
                                                 Module libPlasmaActivities.so.6 from rpm plasma-activities-6.3.5-1.fc42.x86_64
--- SOME MODULES CUT HERE --
                                                 Stack trace of thread 1119019:
                                                 #0  0x00007f6eada8111c __pthread_kill_implementation (libc.so.6 + 0x7311c)
                                                 #1  0x00007f6eada27afe raise (libc.so.6 + 0x19afe)
                                                 #2  0x00007f6eada0f6d0 abort (libc.so.6 + 0x16d0)
                                                 #3  0x00007f6eae01abfc _Z6qAbortv (libQt6Core.so.6 + 0x1abfc)
                                                 #4  0x00007f6eae06e308 _ZL10qt_message9QtMsgTypeRK18QMessageLogContextPKcP13__va_list_tag (libQt6Core.so.6 + 0x6e308)
                                                 #5  0x00007f6eae01c334 _ZNK14QMessageLogger5fatalEPKcz (libQt6Core.so.6 + 0x1c334)
                                                 #6  0x00007f6eae82dc6d _ZL13init_platformRK7QStringS1_S1_RiPPc.cold (libQt6Gui.so.6 + 0x2dc6d)
                                                 #7  0x00007f6eae8e269b _ZN22QGuiApplicationPrivate25createPlatformIntegrationEv (libQt6Gui.so.6 + 0xe269b)
                                                 #8  0x00007f6eae8e3188 _ZN22QGuiApplicationPrivate21createEventDispatcherEv (libQt6Gui.so.6 + 0xe3188)
                                                 #9  0x00007f6eae0fed65 _ZN23QCoreApplicationPrivate4initEv (libQt6Core.so.6 + 0xfed65)
                                                 #10 0x00007f6eae8e746d _ZN22QGuiApplicationPrivate4initEv (libQt6Gui.so.6 + 0xe746d)
                                                 #11 0x00007f6eaf243755 _ZN19QApplicationPrivate4initEv (libQt6Widgets.so.6 + 0x43755)
                                                 #12 0x00007f6eb0f60a55 _ZN4KWin11ApplicationC1ENS0_13OperationModeERiPPc (libkwin.so.6 + 0x160a55)
                                                 #13 0x0000564e568f294b main (/usr/bin/kwin_x11 + 0xc94b)
                                                 #14 0x00007f6eada115f5 __libc_start_call_main (libc.so.6 + 0x35f5)
                                                 #15 0x00007f6eada116a8 __libc_start_main@@GLIBC_2.34 (libc.so.6 + 0x36a8)
                                                 #16 0x0000564e568f7125 _start (/usr/bin/kwin_x11 + 0x11125)

                                                 Stack trace of thread 1119047:
                                                 #0  0x00007f6eada876c2 __syscall_cancel_arch (libc.so.6 + 0x796c2)
                                                 #1  0x00007f6eada7b9da __internal_syscall_cancel (libc.so.6 + 0x6d9da)
                                                 #2  0x00007f6eada7ba24 __syscall_cancel (libc.so.6 + 0x6da24)
                                                 #3  0x00007f6eadaf5176 ppoll (libc.so.6 + 0xe7176)
                                                 #4  0x00007f6eaccf5210 g_main_context_iterate_unlocked.isra.0 (libglib-2.0.so.0 + 0x4a210)
                                                 #5  0x00007f6eaccf52d3 g_main_context_iteration (libglib-2.0.so.0 + 0x4a2d3)
                                                 #6  0x00007f6eae3ff56d _ZN20QEventDispatcherGlib13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE (libQt6Core.so.6 + 0x3ff56d)
                                                 #7  0x00007f6eae103783 _ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE (libQt6Core.so.6 + 0x103783)
                                                 #8  0x00007f6eae2217fd _ZN7QThread4execEv (libQt6Core.so.6 + 0x2217fd)
                                                 #9  0x00007f6eafa27901 _ZN22QDBusConnectionManager3runEv (libQt6DBus.so.6 + 0x20901)
                                                 #10 0x00007f6eae2bdde4 _ZN14QThreadPrivate5startEPv (libQt6Core.so.6 + 0x2bdde4)
                                                 #11 0x00007f6eada7f1d4 start_thread (libc.so.6 + 0x711d4)
                                                 #12 0x00007f6eadb01cec __clone3 (libc.so.6 + 0xf3cec)
                                                 ELF object binary architecture: AMD x86-64
maj 13 10:43:08 demon systemd[1330]: plasma-kwin_x11.service: Main process exited, code=dumped, status=6/ABRT
maj 13 10:43:08 demon systemd[1330]: plasma-kwin_x11.service: Failed with result 'core-dump'.
maj 13 10:43:08 demon systemd[1330]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
2

I enabled debug output from kwin with systemd drop-in file:

$ cat /home/piotr/.config/systemd/user/plasma-kwin_x11.service.d/override.conf
[Service]
Environment="QT_LOGGING_RULES=kwin_*.debug=true"

However after doing this I don’t see any information that might help diagnose the problem in the output of journalctl --user (below).
No warnings nor errors and then timeout:
maj 13 14:38:47 demon systemd[1315]: plasma-kwin_x11.service: start operation timed out. Terminating.

Is there any way to get more detailed debug output from kwin?

Output from `journalctl --user` 
maj 13 14:38:02 demon systemd[1315]: Starting plasma-kwin_x11.service - KDE Window Manager...
maj 13 14:38:02 demon kwin_x11[619433]: kwin_platform_x11_standalone: Has XInput support 2 . 2
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Extensions: shape: 0x "11"  composite: 0x "4"  render: 0x "b"  fixes: 0x "50"  randr: 0x "14"  sync: 0x "31"  damage: 0x  "11"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_xkbcommon: XKB: created compose table from locale en_DK.UTF-8 with path /usr/share/X11/locale/en_US.UTF-8/Compose
maj 13 14:38:02 demon kwin_x11[619433]: kwin_decorations: Trying to load decoration plugin:  "/usr/lib64/qt6/plugins/org.kde.kdecoration3/org.kde.breeze.so"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Attempting to load the OpenGL scene
maj 13 14:38:02 demon kwin_x11[619433]: kwin_platform_x11_standalone: Choosing GLXFBConfig 0xef X visual 0x2fe depth 24 RGBA 8:8:8:0 ZS 0:0 sRGB: 1
maj 13 14:38:02 demon kwin_x11[619433]: kwin_platform_x11_standalone: Global share context format: QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 0, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 0, stencilBufferSize 0, samples -1, swapBehavior QSurfaceFormat::DefaultSwapBehavior, swapInterval 0, colorSpace QColorSpace(), profile  QSurfaceFormat::CompatibilityProfile)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_platform_x11_standalone: Created GLX context with attributes:
                                        Version requested:        true
                                        Version:        3.1
                                        Robust:        false
                                        Reset on video memory purge:        false
                                        Forward compatible:        false
                                        Core profile:        false
                                        Compatibility profile:        false
                                        High priority:        false
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 224 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 1 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 48 VGPRS: 12 Code Size: 3100 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 4 DivergentLoop: 0 (PS, W64)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: OpenGL compositing has been successfully initialized
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "blendchanges"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 208 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 1 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 208 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 1 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 24 VGPRS: 20 Code Size: 316 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 0 DivergentLoop: 0 (PS, W64)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 24 VGPRS: 32 Code Size: 452 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 0 DivergentLoop: 0 (PS, W64)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 188 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "blur"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "colorpicker"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 24 VGPRS: 16 Code Size: 168 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 0 DivergentLoop: 0 (PS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 276 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 1 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 20 Code Size: 452 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 1 DivergentLoop: 0 (PS, W64)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "contrast"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "highlightwindow"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "kscreen"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "outputlocator"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "overview"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "screenedge"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "screenshot"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Effect is not supported:  "screentransform"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Effect is not supported:  "shakecursor"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "slide"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "slidingpopups"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "startupfeedback"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "systembell"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "tileseditor"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "windowview"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded plugin effect:  "zoom"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_platform_x11_standalone: Using FBConfig 0x28e for visual 0x39c
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 224 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 1 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 8 Code Size: 288 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 1 DivergentLoop: 0 (PS, W64)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 8 VGPRS: 8 Code Size: 52 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 0 DivergentLoop: 0 (PS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 16 VGPRS: 16 Code Size: 132 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 16 VGPRS: 4 Code Size: 96 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 1 DivergentLoop: 0 (PS, W64)
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "dialogparent"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "fadingpopups"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "frozenapp"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "fullscreen"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "login"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "logout"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "sessionquit"
maj 13 14:38:02 demon kwin_x11[619433]: kwin_core: Successfully loaded scripted effect:  "windowaperture"
maj 13 14:38:33 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 32 VGPRS: 16 Code Size: 224 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 1 PSOutputs: 0 InlineUniforms: 0 DivergentLoop: 0 (VS, W32)
maj 13 14:38:33 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 40 VGPRS: 8 Code Size: 308 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 1 DivergentLoop: 0 (PS, W64)
maj 13 14:38:33 demon kwin_x11[619433]: kwin_scene_opengl: 0x1: Shader Stats: SGPRS: 16 VGPRS: 4 Code Size: 144 LDS: 0 Scratch: 0 Max Waves: 16 Spilled SGPRs: 0 Spilled VGPRs: 0 PrivMem VGPRs: 0 LSOutputs: 0 HSOutputs: 0 HSPatchOuts: 0 ESOutputs: 0 GSOutputs: 0 VSOutputs: 0 PSOutputs: 1 InlineUniforms: 1 DivergentLoop: 0 (PS, W64)
maj 13 14:38:47 demon systemd[1315]: plasma-kwin_x11.service: start operation timed out. Terminating.
maj 13 14:38:47 demon kwin_x11[619433]: kwin_scene_opengl: Could not delete framebuffer because no context is current
maj 13 14:38:47 demon systemd[1315]: plasma-kwin_x11.service: Main process exited, code=exited, status=15/n/a
maj 13 14:38:47 demon systemd[1315]: plasma-kwin_x11.service: Failed with result 'timeout'.
maj 13 14:38:47 demon systemd[1315]: Failed to start plasma-kwin_x11.service - KDE Window Manager.
maj 13 14:38:47 demon systemd[1315]: plasma-kwin_x11.service: Consumed 362ms CPU time, 85.8M memory peak.
maj 13 14:38:47 demon systemd[1315]: plasma-kwin_x11.service: Scheduled restart job, restart counter is at 1.
3

It turns out there was autologin enabled on remote machine so there was already active plasma (wayland) session when starting plasma-kwin_x11.service. After first logging out from active session with dbus-send --dest=org.kde.Shutdown /Shutdown org.kde.Shutdown.logout and then starting vncserver@:1.service the plasma-kwin_x11.service successfully started.

It’s a pitty kwin_x11 does not emit any interesting information in debug log in this case.