If I set up a Xamarin.Forms Solution in VS 2013 and try to run the iOS Version, it fails because of the following error:
Error 2 Can not resolve reference: /Users/Koray/Library/Caches/Xamarin/mtbs/builds/WalkiOS/aae389efbebffd5cd3625dcf99aad02c/C:/Program Files (x86)/Reference Assemblies/Microsoft/Framework/Xamarin.iOS/v1.0/Facades/System.Collections.Concurrent.dll Walk.iOS
The Xamarin Build Host for Visual Studio is set up and a few weeks ago it worked fine. I can't find anything to fix the error.
The solution is to update Xamarin on both machines to the same version. It should work fine if you do this.
This can happen if you include a library that's not part of Xamarin.iOS. For example, when you Alt-Enter with ReSharper and it auto-adds a reference it thinks will work, when actually, there's no iOS equivalent of it. This happened to me when I had ReSharper add System.Web. The error looked like System.EnterpriseServices.Wrapper, but the actual error was a non-Mono/non-iOS library being referenced by my project. Unfortunately, the error message is effectively wrong. I hope that helps!
Ybrin's answer is also true. I came across this SO article looking for a different answer, and ended up figuring out this alternative answer.