Ok so we have progress.
We deleted aether mod.
Our friend is a dumbass and decided to make an aether portal in nether SOMEHOW.
I think all we have to do now is to reset our nether 
Posts by WinterRose44
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Hi!
Me and my friends have a modded server. I will include the list of mods below.Recently we added a few mods but no one had any trouble joining the game until yesterday. Everytime my friend tries to join our server his game crashes. I don't think it's a problem with mods - he can create singleplayer worlds normally and play on them. All versions of mods in his folder are the same as the rest of players.
I attached both the error message and our mod list in the "attachments"
Hopefully you will have better ideas of what to do than we do!