Comment 15 for bug 1965219

Thanks for that. The log in comment #12 shows these errors which may explain it:

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Mär 22 01:57:12 aletheia systemd[1]: systemd-logind.service: Watchdog timeout (limit 3min)!
Mär 22 01:57:12 aletheia systemd[1]: systemd-logind.service: Killing process 1501 (systemd-logind) with signal SIGABRT.
...
Mär 22 01:57:12 aletheia systemd[1]: systemd-logind.service: Main process exited, code=killed, status=6/ABRT
...
Mär 22 01:57:13 aletheia /usr/lib/gdm3/gdm-x-session[2486]: (EE)
Mär 22 01:57:13 aletheia /usr/lib/gdm3/gdm-x-session[2486]: Fatal server error:
Mär 22 01:57:13 aletheia /usr/lib/gdm3/gdm-x-session[2486]: (EE) systemd-logind disappeared (stopped/restarted?)
...
Mär 22 01:57:13 aletheia systemd-logind[95934]: Failed to restore VT, ignoring: Bad file descriptor
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And yes we expect "Fn+Ctrl+Alt+F??" to fail in this case because the processes which implement that combo have failed.

To be sure we're on the right track, please reboot, wait for the bug to occur again, and then redo:

  journalctl -b-1 > prevboot.txt