I just upgraded on Neon. Getting a similar thing. I see the login splash screen. I enter my password, the screen goes dark, then I get returned to the login screen.
I used PUTTY to look around. The kernel is running normally (6.5.0.21 generic).
I tried a login and got this from /var/log/syslog
jim@linux:/var/log$ tail /var/log/syslog
Feb 28 09:40:07 linux dbus-daemon[1521]: [session uid=118 pid=1521] Activating via systemd: service name=‘org.freedesktop.impl.portal.desktop.gtk’ unit=‘xdg-desktop-portal-gtk.service’ requested by ‘:1.10’ (uid=118 pid=2516 comm="/usr/libexec/xdg-desktop-portal " label=“unconfined”)
Feb 28 09:40:07 linux systemd[1506]: Starting Portal service (GTK/GNOME implementation)…
Feb 28 09:40:07 linux xdg-desktop-por[2521]: cannot open display:
Feb 28 09:40:07 linux systemd[1506]: xdg-desktop-portal-gtk.service: Main process exited, code=exited, status=1/FAILURE
Feb 28 09:40:07 linux systemd[1506]: xdg-desktop-portal-gtk.service: Failed with result ‘exit-code’.
Feb 28 09:40:07 linux systemd[1506]: Failed to start Portal service (GTK/GNOME implementation).
Feb 28 09:40:07 linux sddm-greeter-qt6[2510]: Adding view for “VGA-0” QRect(0,0 1920x1200)
Feb 28 09:40:07 linux sddm-greeter-qt6[2510]: Hunspell dictionary is missing for “en_CA”. Search paths QList(“/usr/share/qt6/qtvirtualkeyboard/hunspell”, “/usr/share/hunspell”, “/usr/share/myspell/dicts”)
Feb 28 09:40:07 linux sddm-greeter-qt6[2510]: Message received from daemon: Capabilities
Feb 28 09:40:07 linux sddm-greeter-qt6[2510]: Message received from daemon: HostName
jim@linux:/var/log$