When running UltraVNC as a service (in which case it normally serves the console session), and the console session is logged in with unvc running (e.g. session 1), and then a remote user connects to the machine via RDP and obtains another session (e.g. session 2), and that remote user runs within session 2 `tscon.exe 1` to switch their remote connection to session 1, then UltraVNC hangs forever until the service is restarted.
I've tested this on Server 2008 through Server 2012 R2 through Windows 10.
I'm not saying what the correct behavior of UltraVNC should be (e.g. whether it should continue to serve session 1, or whether it should attach itself to the new console session, session 3), but it shouldn't hang forever.
Stopping & starting the service via the settings panel usually restarts it, but sometimes a reboot is required.