If you have a Realms account with 3 open worlds available.  You then go to Replace a world with one of your local ones.  The world replace will successfully export the world.  On the screen where it starts to Initialize the upload, it will freeze there.  After a few moments, you will get a pop-up box that just says "An error occurred" and nothing else.  

This happens on Pocket Edition as well as the latest on Windows 10.  Creating a new Realm world and paying for it works fine.  using the 3 available slots in the Realm does not.