We've made a iOS/Android/WP8.1 app, and when submitting the appxbundle to the Store, we get about a hundred validation errors like these, and all are from XLabs.Platform.WP8.dll:
This API is not supported for this application type - Api=System.IO.IsolatedStorage.IsolatedStorageFile. Module=MSCORLIB, PUBLICKEYTOKEN=7CEC85D7BEA7798E. File=XLabs.Platform.WP8.dll.
This API is not supported for this application type - Api=System.Windows.Threading.DispatcherTimer.Stop. Module=SYSTEM.WINDOWS, PUBLICKEYTOKEN=7CEC85D7BEA7798E. File=XLabs.Platform.WP8.dll.
This API is not supported for this application type - Api=System.String.Format(System.String,System.Object). Module=MSCORLIB, PUBLICKEYTOKEN=7CEC85D7BEA7798E. File=XLabs.Platform.WP8.dll.
We are doing nothing special with these APIs; the app is very simple. It sounds like we're perhaps building the app in the wrong way, or targetting the wrong Windows Phone version, or something like that.
The app works fine when deployed directly to a developer phone.