We have had an ongoing problem affecting all of our terminal servers in a TS farm at random times and not all at once. It doesn't seem to be a problem with the individual servers but a roaming issue that causes the entire farm to stop serving sessions. There are a couple event ID's that show up in the RDS on the server:
Error 36: Microsoft-Windows-TerminalServices-LocalSessionManager: An error occurred when transitioning from DisconnectedLoggedOn in response to EvConnected. (ErrorCode 0x80070102)
Error 1152: Microsoft-Windows-TerminalServices-RemoteConnectionManager: Failed to create KVP sessions string. Error Code 0x8007007A
The only way to allow the TS farm to start allowing connections is to reboot the problem server at that time.