Can't start builds or login

As of this morning, I am unabe to trigger builds on TeamCity - it just says "queueing" and then nothing happens. I have rebooted the server.

I logged out and now cannot log back in - when I try the request to "loginSubmit.html", this eventually returns with an xml snippet:

<response><errors><error id="publicKeyExpired">...hex code removed..</error></errors></response>

Nothing is jumping out at me from the server logs, and users who are still logged in can navigate ok (just not start new builds).