Thank you for the quick response.
I'm sorry for the work I generated. After reporting the bug, I tried another
display / window manager, respectively, and managed my system into a
non-working GUI.
I then did a backup and reinstalled Linux, now cinnamon is working well.
So you can close the bug in
Public bug reported:
On my Ubuntu 20.04.3 LTS, I have reproducible freezes of gdm3
(3.36.3-0ubuntu0.20.04.4) caused by third-party applications. The only thing I
found describing my problem was https://unix.stackexchange.com/a/39855 (passage
*GUI Glitches Causing Unresponsive WM or X11/Wayland*