What a cool title, eh? I don't think so either, but this is an example of one of the many errors the OCS edge server may throw for the simple fact that the back end OCS server's services are in fact stopped.
I was working debugging external Web conferencing issues for a customer when overnight external Communicator access stopped. Telnet to port 443 revealed the port was open and listening. Checking the user in AD Users&Computers revealed the user was enabled, and so was external access on the edge server. The edge server confirmed services were up. Tracing gave me the cool error message which is the title of this post.
On a hunch I checked if the back end services were started, but they weren't. Problem solved, this time around however this error can pop up for a host of issues, including some not so obvious ones like services not being started......
Other authentication issues can creep up when the back end is configured to use TLS instead of MTLS. This will break the encrypted comms between the edge and the back end, however tracing will also reveal authentication errors.