I renewed my Lync certificates recently and after applying them, response groups stopped working correctly. Basically, a call would come in, the agent would attempt to answer it, and upon doing so the call would immediately disconnect. The call would continue to ring and bounce between agents until the queue naturally timed out.
The strange thing is that the Lync logs showed nothing remarkable: no errors, no warnings. I did a SIP trace on the Lync Logging Tool, with no luck. On a whim, I restarted the Lync Server Response Group service and after doing that, response groups started working again!
Going back through the log, I can see where Lync complained about being unable to connect to the match maker service, with a yellow warning, about the time I changed the certificate. There were no further errors or warnings. After I restarted the service, I saw a flood of information messages about connecting to and updating things with the match maker service, so I’m guessing (truly, this is a guess) that might be the culprit.
Moral of the story: update Lync certificates after hours and then reboot (or at least restart all Lync services) to be safe.