I am having the same problem after updating to Xamarin studio 6. The update removed my monogame addin which I reinstalled - version 3.5.1.1679
My font that compiled fine on what I had before (don’t remember the version) now no longer compiles. Google searches aren’t really throwing fruit into my basket.
IIRC, I gave up on trying to get stuff compiling properly in Xamarin and instead reverse engineered the .app protocol so that I could deploy to OSX directly from Windows without having to actually recompile my project on a Mac.