To solve the issue I now have deinstalled everthing from plasma and kde and reinstalled again Now plasma is running without crash or black screen at startup.
I suspect the package plasma-calendar-addons beeing the cause - but I not sure as I have not tested it again. I have read something about plasma-calendar issues here https://www.reddit.com/r/kde/comments/isi2s6/event_calendar_making_latest_plasma_and_qt_crash/ and this is the only package I remember which was installed before but I didn't install again now.
Hope this help.
Regards,
trupf
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