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.
