Even after manually copying the world-folder to another device, trying again with a freshly installed iPad, freshly installed MineCraft, nothing worked. It would go through the upload all the way to 100% and then fail with the dreaded "An error occurred" dialog box.
Eduardo Lazcano further down, commented the solution that worked! Look at his comment from 13th April 2023.
I used the Files-app on iOS and emptied the content of this folder:
fffrrrooonnnttt
I have had this problem for months now, on iOS.
Even after manually copying the world-folder to another device, trying again with a freshly installed iPad, freshly installed MineCraft, nothing worked. It would go through the upload all the way to 100% and then fail with the dreaded "An error occurred" dialog box.
Eduardo Lazcano further down, commented the solution that worked! Look at his comment from 13th April 2023.
I used the Files-app on iOS and emptied the content of this folder:
/MineCraft/games/com.mojang/minecraftWorlds/[WORLD NAME]/resource_packs/
The upload size went from 128,9Mb to ~45Mb, and went through to our Realms instance on the first try!