Can't upload world to Realms even on 1.16.1

I am trying to upload a world to Minecraft Realms, but I am unable to do so.

I am currently trying to do so in 1.16.1, which should have solved the issue, but it's not working yet. I am able to join worlds currently there, but not upload new worlds.

I get "Upload failed (-1)" everytime, at random percentages of upload, as seen in the screenshot.