I just updated Xamarin Studio to the latest version (Xamarin.Android Version: 4.6.4 (Business Edition)) and I'm receiving a lot of warnings after the update..:
/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets: Warning: Reference 'mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=xxxxx' not resolved (TrackandTrace.Droid)
/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets: Warning: Reference 'Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=xxxx' not resolved (TrackandTrace.Droid)
I already tried to remove the reference and re-add it without any effect.
Update: Even if i create a new project I receive 2 errors right from the start.. These errors make unable to build android projects.
I also experienced that after upgrading(I believe you tried in release mode) try to change your appropriate architectures in your applications properties (if not sure, select all 3)