I too get this error This is what I found on Google (gotta love GOOGLE)
http://www.xs4all.nl/~zweije/xauth-7.html
copy and past this snip, just replace command with the command you want to run, works great!
su - -c "exec env DISPLAY='$DISPLAY' \
XAUTHORITY='${XAUTHORITY-$HOME/.Xauthority}' \
command"
thanks Mark works fine I've to investigate whu it appears suddenly and not from the installation .....
Franck
