Xamarin, Invalid or Corrupt License
If you get this message when developing Android apps with Xamarin, "Invalid or Corrupt License" it does not necessary means that it is something wrong with the licens. In my case it was API level 19 that caused the problems. After I unistalled it from SDK Manager I have not seen the message. This seems to be a common problem, I have read about other developers having the same problem with API level 19.