I was in the Wayland session, and after seemingly successful update and reboot it was as if the password was not recognized at login; no such message, just the password field gets empty after "enter".

Trying to log in X session, the password seems accepted but the process freezes during the following splash ("welcome to Plasma").

How to troubleshoot this? Can it be repaired from a live USB session of October ISO? (I don't have a working KaOS now, but I have Kubuntu LTS and Windows 10 working.)

Same problems here this morning. After an full update at 09.00hrs I had the same problem. Spent a few hours thinking it was my theme stopping the display.
Eventually tried pacman -Syu at around 13.00hs and another batch of files installed. After that it booted OK.
So try ctl + alt +F2 and login as root and then try another update.

    An unforeseen issue happened with moving plasma 5.23.2 (build on a new gcc 11.20 & binutils 2.37 based toolchain) without moving said new toolchain too. So the new toolchain moved today, to fix any kwin issues.
    Logging in in tty2 as your regular user--not needed to login as root--- (no X or wayland) is not effected, though logging in at X should work ok, just the windows won't work as should.
    So either update as mentioned above in tty2, or try an X session.

      nezzie pacman -Syu in tty brought new update and fixed this. Thanks.

      Solved.

        demm Could such mishaps be voided by doing the update through tty2 in the first place?

          No, normally such things are caught while in the [build] repo, just my mistake to move the Plasma desktop build on a new toolchain. This time, the [build] repo was more stable then stable......
          But, won't happen again, next toolchain (usually every 6 months a new toolchain), nothing will move from [build] until the toolchain it was build on is ready to move.