Thanks for your help!
Problem was in the app.cs and the IoC registration Code like Stuart assumed. I renamed the two Implementation Classes and removed the 'Service' ending. So the Code now casts like this:
_syncService = (Type1)syncService;
and the Registration looks like that:
public override void Initialize()
{
CreatableTypes()
.EndingWith("Service")
.AsInterfaces()
.RegisterAsLazySingleton();
if(Config.Config.type == ControlType.Type1)
{
Mvx.RegisterType<IControlService, Type1>();
}
else if (Config.Config.type == ControlType.Type2)
{
Mvx.RegisterType<IControlService, Type2>();
}
RegisterAppStart<ViewModels.FirstViewModel>();
}