Can't reboot nor shutdown

[Bzzzz <lazyvirus@gmx.com>]

Hi list,

on my latptop (lightdm + XFCE) I can't reboot nor shutdown properly;
each command ends with a black screen with a blinking cursor at top
left and that's all. I waited several times more than 5 minutes,
hoping some timeout was there, but nope; stopping with the power
button isn't very good for the HD.

On my SOS system (almost the same setup), where I only login under
root, it works (but the system's not up to date).

The only line I found in /var/log/syslog & messages it this one
(it is the last one before boot log lines):

Sep  3 04:30:07 msi kernel: [239215.267029] cupsd[2445]: segfault at 0
ip 00007f28d5e7bd54 sp 00007fff10207b00 error 4 in
libavahi-client.so.3.2.9[7f 28d5e76000+10000]

Does anybody knows where it could come from? (line above or missing
perms?)