A friend an me played on a Realm world that I hosted without problems. We now wanted him to host our world on his Realm.
We tried downloading the world from my Realm as local world, exporting it and importing it at my friends PC with his Minecraft account on his Realm (without loading the world previously as local world). This produced a Message "Error occurred" or just hangs (60 minutes and more) at initializing upload, see attached screenshot.
We also tried to load and play the world local (with both our accounts) before importing it on my friends Realm and got the same error. We noticed that we both had my characters last location and equipment when used the world locally.
UPDATE:
After deleting all local games and loading our world multiple times we could now both join the Realm of my friend. We both started from the same location and with the same equipment (It was the last location of my character on my old Realm.)
Both of you starting from the same location, etc may be due to realm ownership change. However in any case that is a different issue than the original, which now seems to be solved per the updated description.