I've also had the above issues, but like IonicEcko my world successfully uploaded. I have 2 realms. Before 1.16.20, the first realm would finish the level export and then sit on the "Initializing Upload" part for a while before giving an error. The second realm would give the error the first realm gave, but almost immediately. On 1.16.20 the second realm now does what the first realm did before the fix and still fails. The first realm now stays on the "Initializing Upload" part even longer than before, then gives the same error, but the world is there when I get on the realm. If you have a realm that has a similar behavior to my first realm then try uploading your world now, it more than likely will work.
Mark Erickson
I've also had the above issues, but like IonicEcko my world successfully uploaded. I have 2 realms. Before 1.16.20, the first realm would finish the level export and then sit on the "Initializing Upload" part for a while before giving an error. The second realm would give the error the first realm gave, but almost immediately. On 1.16.20 the second realm now does what the first realm did before the fix and still fails. The first realm now stays on the "Initializing Upload" part even longer than before, then gives the same error, but the world is there when I get on the realm. If you have a realm that has a similar behavior to my first realm then try uploading your world now, it more than likely will work.