Today we noticed a number of machines grinding to a halt. Attempting to register the machines has resulting in the following error:

Unable to register with the server at: https://server.
RegistrationManager - Network error connecting to server: The request was aborted: Could not create SSL/TLS secure channel.

At first glance I assumed it was just an angry old build, but they've been popping up all over the place. I now suspect that it has something to do with a certificate that's about to expire on one of the servers in the User Source, but I'm not sure. Can I remove a server from the User Source authentication list without killing my user associations? There's two in the list, so removing the old one should hurt anything right?

Regardless, is this even the issue? Has anyone else seen it?