If you started your ssh session with the X11 forwarding enabled but with the following message:
/usr/bin/X11/xauth: error in locking authority file ...
There is a chance the .Xauthority in your home-directory has wrong permissions ( should be -rw-------, and owned by you; check with ls -al, solve with chmod/chown ) or that your diskspace for your home directory is full. If this is the case try cleaning up your home directory.
