Similar issue on Arch.
Just a flashing “_” cursor on black screen when SDDM should show up.
I tried what was suggested here:
Did not work, not a Issue with the Breeze theme here.
The KDE start from the command line with /usr/lib/plasma-dbus-run-session-if-needed /usr/bin/startplasma-wayland
complained about unable to get display or something similar.
I deactivated SDDM systemctl disable sddm.service and restarted, now the display complain is gone but I still get:
org.kde.startup: "kdeinit5_shutdown" QList() exited with code 255
startplasma-wayland: Shuting down...
startplasmacompositor: Shuting down...
startplasmacompositor: Done.
And that’s it.
Used timeshift to revert to 6.2.0 in fact I’m slightly more back now then where I was yesterday.