I’ve noticed that if I’m away from my PC for a while (generally over an hour), Visual Studio loses the connection to the Mac Build Host. When I try to reconnect via the Visual Studio Tools/Option/Xamarin/iOS Configure button, it won’t reconnect. The Mac build host is shown as unreachable and Diagnose fails trying to talk to the Mac.
Restarting Visual Studio doesn’t do it, you have to close down then reopen Xamarin Studio. It’s 4.0.12 (build 3). Then you can reconnect from Visual Studio. Not a big bug, just a minor PITA!