To me that sounds like permission issue from server side... In which case you should be able to find some entry relevant to the issue in the server's syslogs.
Is it the same whether you ssh as user or root?
I recommend you looking into 'xauth'
I have never had to do this myself, but it seems like worth trying:
xauth extract - $DISPLAY | ssh server xauth merge -
which will pass the xauth magic cookie to the server. See http://www.linuxsecurity.com/content/view/124631/177/