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Update:
I did what I wanted, but I had to turn both servers to Vanilla first and then reinstall PaperMC to both, still don't know how to do it while keeping them both in PaperMC at all times.
Perhaps someone can reply with such a fix for future readers.
A friend of mine has an Aternos server and I want to make a replica, I downloaded the "world" "world_nether" and "world_end" files from his server and tried to upload them on mine. First I changed it from Vanilla to PaperMC, as my friend's server was and downloaded the same plugins to make sure. Then I uploaded "world" with no problems at all, but "world_nether" was not even generated so I could not upload it (as far as I know, didn't find anything on how to upload "world_nether" when it doesn't show up in worlds). To fix that I started the server and went to Nether just so it generates it. I turned off the server and tried to upload "world_nether" but it says that file is not valid.
On a similar Thread that is now closed a Helper said to download "world_nether" from the server, delete the "DIM-1" file from "world_nether" and replace it with the new one, but that was to fix an issue of uploading a vanilla Nether to a PaperMC server, I don't think this helps me.
What should I do?
Here is the other Thread: Can´t upload nether.
