Some X Clients work with ssh X forwarding, some not

It seems like a really strange issue: you connected to a unix machine (running opensshd) with ssh -X, you can run X clients like xterm there, but some applications fail to start with the following error message:

X11 connection rejected because of wrong authentication.
Unable to continue communications with the X display.
Please ensure that the X Server is still running, and the
connection to it is still valid.
But the solution is very simple: the client tries to connect localhost instead of the machine, for which the auth may not be set. OpenSSH has an server option to configure this behavoir. In sshd_config, change the (usually commented) line
#X11UseLocalhost yes
to
X11UseLocalhost no
and restart the sshd. Now this problem should not appear anymore.