As of today at noon, my iOS project in a Xamarin Forms shared library project, Visual Studio 2013 Update 3 stopped working, with this error.
This version of Visual Studio is unable to open the following projects. The project types may not be installed or this version of Visual Studio may not support them.
For more information on enabling these project types or otherwise migrating your assets, please see the details in the "Migration Report" displayed after clicking OK.
The weird thing is that I did not manually update anything, and things were working this morning. I restarted Visual Studio, and then nothing works anymore.