After Updating Xamarin.Forms - App doesn't start

Hello,
I played around with Xamarin a little bit but now, after updating to Xamarin with NuGet, my App doesn't start anymore
I use Visual Studio Ultimate 2013 and Xamarin Forms 1.2.3.6257

While Debugging (on Android Phone) it stops here:

(27241): Missing method Xamarin.Forms.FileImageSource::op_Implicit(string) in assembly 
/storage/emulated/0/Android/data/Locations.Android/files/.__override__/Xamarin.Forms.Core.dll, referenced in assembly 
/storage/emulated/0/Android/data/Locations.Android/files/.__override__/Locations.dll

The Exception I get is:

System.TypeInitializationException: An exception was thrown by the type initializer for Location.App

If you need further information please tell me.
Thanks