Subj occurs quite often, non-peridocally at some of my agents which have hanged builds. Builds are hanging because of some third-party code analysis tool hangs, so it's not a TC fault.
But it is not only agent problem. When I see this exception at my agent (it uses agent.bat start to start TC) I know if I reboot/stop TC service of any other agent it won't connect to TC back with the same message in log/console.
What helps is TC server restart. Disconnected agents are being back and hanged builds are disappearing from that agents. Before restart they say that hanged builds cannot be stopped.
My guess is something happens to TC RPC service or something with agent. I turned on debug mode for rpc, nothing usefull happened insode the log, just bunch of messages from agents who were connected and no any info from disconnected agents. On the agent site I didn't see any useful inside the log file either. Even if I do enabling debug for xml rpc at log4j configuration.
For this time it works well, I've recently restarted server. I had this problem with 8.0.6 and then with 8.1.2, 8.1.3 versions. With 7.x I had not