Maybe this will help others having this issue.
For a while when I would build with VS I'd get exception: System.Net.WebException: The remote server returned an error: (500) Internal Server Error.
Using Fiddler I wasn't able to see the request/response, only Bad Request (Invalid host). To fix this see: https://groups.google.com/forum/#!topic/httpfiddler/q_WxDoAZe-o
and in Fiddler do: prefs set fiddler.network.SetHostHeaderFromURL false
That allowed me to see the response and in my case I had an image in Resources that was project linked. So the ContentCopier part of the build failed saying the source file didn't exist. Once I added the image as a non linked resource, I could build and run again.
The mtbserver.log shows it trying to copy but doesn't indicate there was a problem.
With help from @RossDargan.