Try to reinstall your server
So i found out the problem was with one of the mods i was using. Thanks a lot for helping though 
Posts by Colonel_Yeet
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Try to reinstall your server
Still doesn't work
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Please try to delete your playerfile on https://aternos.org/files/world/playerdata
It didn't work. Any other suggestions?
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Every time i try to connect to the modded server i made, it gives me a message saying "Connection Lost: Internal Server Error" It loads the terrain and everything then removes me.