I don't typically relate that to an expired OAuth cert, however there has been a lot of changes in the last year with security updates and Exchange, so you absolutory need to renew that cert.
The latest Jan 2023 SU for example:
So I recently inherited an older Exchange 2013 Standard server (fully patched)
I noticed that the Microsoft Exchange Server Auth Certificate has been expired for a long time.
There are no complaints or any issues reported by the users but I have noticed a few things myself and I wonder if they are related.
I know how to renew this certificate but I wonder if anyone knows what kind of symptoms you would see if such a certificate has expired?
What I noticed myself is some users, not all, get a message that the server is unavailable if you want to set an Out of office reply from Outlook. Setting Out of office using OWA works fine.
I also noticed that in Outlook, some users get mailboxes automapped even though they don't even have Full Access anymore. If you create a new Outlook profile these mailboxes show up again but when viewing ECP no Full Access permissions are listed...
Could these be related?