So apparently the culprit was a line xset -b
in /etc/gdm3/PreSession/Default , added long ago to disable the default "bell". Removing it allows logging in normally, and re-adding it restores the erroneous behavior. So probably the bug is that a. Such a configuration causes complete impossibility to login. b. No information about the error is found anywhere (the file /etc/gdm3/PreSession/Default says errors will be logged to .xsession-errors, but that file contained no useful information). Regards, Itai.