Why does XForwarding not work anymore?

If you're getting messages like these while trying to forward an X-session:

user@laptop:~/bin$ ssh -X user@login.ee.ethz.ch
user@login.ee.ethz.ch's password:
Last login: Mon Apr  2 13:39:11 2007 from vpn-global-083-SunOS 5.8 Generic_117350-44 UNW,Sun-Blade-1500
user@tardis-a05:~>matlab
X11 connection rejected because of wrong authentication.
X connection to localhost:11.0 broken (explicit kill or server shutdown).

or like these:

Xlib: connection to "localhost:10.0" refused by server
Xlib: PuTTY X11 proxy: MIT-MAGIC-COOKIE-1 data did not match
Warning: Unable to open display localhost:10.0
(mit-magic-cookie)

Log in first via ssh and try the command xauth-fix. If this doesn't help, let us know at support@ee.ethz.ch

CategoryLXCL