I downloaded my realm's world a few days before the Nether Update came out (1.16) I made some minor edits to the world. One or two days after the update came out I attempted to re-upload the world. I tried both "replacing world" and "play on realm" options to upload the world to the realm.

Initially, for the first 10 times trying to upload it would finish, then a bar would appear at the top of the screen saying the upload was a success, then an error prompt would appear saying "Error: An error has occurred".

After trying again a day or two later it would not come up the error prompt. It would stay on "Initializing Upload". So far I have only left it on this screen for 20 minutes without any change. I would stop the tests as I would need to use my phone.

TLDR: Cannot upload the world to realms, gets stuck on Initializing Upload screen.