Comment # 3 on bug 1084798 from Michal Srb

(In reply to filippos Filippos from comment #2)
> filippos@FOSTW:~/tmp> echo $XAUTHORITY
> /tmp/xauth-1000-_0

That is unusual place for xauthority file.

> filippos@FOSTW:~/tmp> xauth list
> xauth:  file /tmp/xauth-1000-_0 does not exist

Looks like something deleted it meanwhile. Maybe something is cleaning your
/tmp/ regularly. Or whatever created it incorrectly determined that it is no
longer needed.

Which display manager are you using? You can find out using this command:
update-alternatives --display default-displaymanager