Server closes immediately after a player joins

Made a modded server with friends, have played on it dozens of hours, and it's worked pretty well but its developed a recent issue. The server opens great, nothing wrong there, but the minute someone joins it closes down the server immediately.

I've run a couple of vanilla aternos servers before, juryrigged myself some makeshift modpacks, and othersuch, so I know how to read logs somewhat. The thing thats annoying about this issue is that it doesn't show anything wrong in the log.

Is this a problem with the world, with aternos servers, or multiplayer in general?

Oh, nevermind log is finally giving me something.