While trying to launch claws-mail in my remote ssh-forwarded display I got an:
X11 connection rejected because of wrong authentication error message.
I realized then that while DISPLAY was configured correctly to point localhost:10.0 I was using another user in the screen session, not the one used to ssh in. After some googling for the message seems the usual culprits for this were low disc space (!) and disabled X11 forwarding, which were not my case. There were mentions to ~/.Xauthority permissions, but you don't have such file when you su to another user. So xauth came to my rescue: on the user which logs in you can list authorizations:
$ xauth list busgosu/unix:0 MIT-MAGIC-COOKIE-1
localhost.localdomain/unix:0 MIT-MAGIC-COOKIE-1 busgosu/unix:10 MIT-MAGIC-COOKIE-1
And in the user you su-ed to, and which doesn't have the file:
$ xauth xauth: creating new authority file /home/otheruser/.Xauthority Using authority file /home/otheruser/.Xauthority xauth> add busgosu/unix:10 MIT-MAGIC-COOKIE-1
xauth> exit Writing authority file /home/otheruser/.Xauthority $
And you're done, with the same authorization now X11 forwarding works for the other user too :-).