After updating the Skype for Business Persistent Group Chat server I noticed that the Skype client would give me an error message:
Old messages would not load, and trying to enter new ones would simply time out.
Suspecting a back-end connection error (not able to retrieve messages from DB) I ran the Test-CsDatabase -DatabaseType PersistentChat cmdlet, but it completed with success.
Looking into Event Viewer on the Persistent Chat server showed no errors.
A quick search online gave few results, but this blog post had some similarities. Going with the suggestion that this might simply be client related I restarted the client- and voilá, it started working again! The client had been running during server update and probably just needed a kick in the “behind”.