Same problem but none of the world work for me.
I had a Vanilla 1.15.2 server on Aternos and now want to transfer it to Minehut.
Sadly, I can not even transfer my overworld. Upload button on the web site goes to 100% and then nothing happens, no world get added to my Minehut server. Another method (/ul world + worldname + file io link) does not work because my upload always fails on file io. Anybody had the same issue and knows of any alternatives?