I have attempted to upload a world file multiple times to different slots. Each time after it prepares the world it starts the process and sits at 0% after a couple of minutes it fails with error (-1) still at 0%.
Related issues
Comments
Avoma
migrated
This appears to be a duplicate of REALMS-2159, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.