I'm making a modded server, with 300 mods. The server works like a charm but when I try to stop it I get this issue. Is this a big problem that can break my game? Can I still play it with this error? How can I fix it and would my world get corrupted from this?

My modpack is an edited version of the modpack called BetterMC. I'm only playing with my gf, so 2 players. I think it could be because of the server configs of BetterMC's serverpack, since I deleted some mods from the original modpack. Is there a way to detect the configs that are causing this problem so that I can get rid of them?
Here's a photo of the error:

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