Fixing X11 connection rejected because of wrong authentication

You get either "X11 connection uses different authentication protocol" or "X11 connection rejected because of wrong authentication".

The thing is, this is a very generic error message that will take you to multiple very different fixes.

Many telling you to set a variable here and change a setting there.

In my case, it was because the SSH server had a configuration under /etc/ssh/sshrc.

I found the answer here.

In my case, sshrc only had comments as I had stopped to use the file, so I just removed the config file and ssh -X user@server xclock worked again.