update, I've checked every mod version individually and they're all the same (except for some client mods like sodium and inventoryhud) which isn't on the server.
Posts by teefssi
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Oh, It's time to check if every mod version is the same then. But what if it isn't?
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TLauncher and the official Minecraft launcher have been used aswell.
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So, I've started a fabric server (around 90+) mods and has started trying it. At first, I thought this error would be gone after a reconnect, but it still repeats even after so much reconnecting. I've looked up the problem, and apparently it gets fixed when you change your username. But then after you close your client and reconnect to the server, You get the same thing, which is very annoying cause data wouldn't save and we'd have to start all over again. I'd appreciate it a lot if I could be helped!

here's the mclog: