The following message may show up the first time you SSH login to a X11 forwarding enabled machine :
/usr/bin/xauth: file /home/%user/.Xauthority does not exist
In order to workaround this issue, just do the following :
1. Create an empty file for the concerned user :
touch /home/%user/.Xauthority
2. Change the user/group permission to match the concerned user :
chown user:group /home/%user/.Xauthority