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
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.