This was the issue. I feel stupid for not checking it.

Good morning,
We came in this morning to find that none of our accounts were pulling tickets; it said our tokens had expired, even though they were just renewed last week. Now, when I attempt to renew them, I get "invalid_client" as a response. I upgraded the plugin in an attempt to resolve this, but no luck. Any idea what's going on here?

I think this would be a fantastic addition that could cut down common 'simple' requests - or even just to reply and route tickets to the appropriate place.