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.
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…