# cat .xsession-errors
/etc/gdm/PreSession/Default: Registering your session with wtmp and utmp
/etc/gdm/PreSession/Default: running: /usr/bin/sessreg -a -w /var/log/wtmp -u /var/run/utmp -x "/var/lib/gdm/:20.Xservers" -h "" -l ":20" "rui"
/etc/gdm/Xsession: Beginning session setup...
_XSERVTransSocketOpenCOTSServer: Unable to open socket for inet6
_XSERVTransOpen: transport open failed for inet6/ubuntubox:1
_XSERVTransMakeAllCOTSServerListeners: failed to open listener for inet6
_XSERVTransSocketUNIXCreateListener: ...SocketCreateListener() failed
_XSERVTransMakeAllCOTSServerListeners: server already running

Fatal server error:
Cannot establish any listening sockets - Make sure an X server isn't already running

(gnome-session:20828): Gtk-WARNING **: cannot open display: