Failed to change active application on server.

I can 'Pair' with my Mac and PC, but i'm getting this error when I try and run the emulator

'Server command 'ChangeActiveApplication': failed to change active application on server Command execution task ended with exception Exception System.Net.WebException: The remote server returned an error: (500) Internal Server Error.'

I'm using Visual Studio 2013. Anybody had the same issue?