So finally managed to get a new SSL certificate up on the site server, so the error should be gone for now. Let me know if you see it again
For those interested, it appears that our server runs on an old version of linux that is no longer supported, and cannot be upgraded either, so I will need to standup a new server in the near future, and then transfer everything over to it instead.
For now at least the new cert will give us a temporary stay of execution, but Ill need to get that new server setup within the next month or so.