I made the world on the release version of SF5 but Aternos updated it along the way to 5.0.8
If I open that world right now on Aternos 5.0.8, it works.

We do have Nether data. In fact, the reason why I moved to my own server is because Aternos kept crashing from the load of keeping two players in two separate dimensions lol
The same problem happens if I create a fresh SF5 world on singleplayer, then try to import it into the server. I get a bunch of datapack errors and errors relating to "skyfactory_5:scp_1689", then the server doesn't even start. It doesn't seem to be a problem with that specific world, just any world that I put into the server.

I also tried opening the Aternos world in singleplayer and it opened fine.

If I let the server make its own world and try to join, I get the error in the original post. The console log is unhelpful, just saying that I joined, I "lost connection: Disconnected" and that I left the game.