Cannot start KDE under X

[cfgauss]

It seems to boot normally. (All green [ok]s.) There are no errors in Xorg.0.log and there is no ~/.xsession-errors. When it boots into a bash terminal startx produces an error: KWin is unstable. It seems to have crashed several times in a row. You can select another window manager to run. But the only choice it gives is kwin_x11. Executing kwin_x11 complains no Qt platform could be initialized. I also have had problems with installing Qt 6.

Any hints on solving this will be gratefully received.