Am 20.11.21 um 16:24 schrieb Andrei Borzenkov:
Full output of "journalctl -b" and full Xorg.log may give some hints.
Xorg.log I didn't find anything useful. Checking journalctl -b ...
and finally it surely was my stupidity :-(
I added some users manually when migrating from my old to the new system and finally lightdm's var folder where owned by another user.
Sorry for the noise.