X11 connection rejected because of wrong authentication – X11 forwarding suddenly fails

After ages of flawless X11 forwarding over SSH, today I started getting authentication errors and couldn’t even get a remote xterm to display locally over my ssh tunnel.

Weird!

I tried ssh -Y, ssh -X and changes in sshd_conf on the remote server and ssh_conf locally, even though I knew that nothing had changed except a few patches to unrelated software on the local machine. Of course that didn’t help.

I ran xauth on the remote server, no indication of any errors.

It turned out that the remote /home filesystem was out of space, and this prevented the ssh X11 forwarding from working properly. I write this as a note-to-self, as it could happen again…

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