Describe the bug
I load the game, and I have the backpack installed, A few minutes fly by, crashes, so I look into the server crash log and suspected your backpack cause it's not mesh so it's sus

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To Reproduce
Steps to reproduce the behavior:

1. Load New World with backpack installed
2. enjoy your time
3. keep enjoying your walks like rainbows and lollipops across the land
4. Fall

Expected behavior
Minecraft

Screenshots
I can't post my browsing history here so here is my FBI EXTERNAL LINK!
https://paste.gg/p/anonymous/7a23e6f6bd264f97ab85e075117119dd
Versions
yippe ki-yay it was a sophisticated backpacks-1.20.1-3.19.1.962.jar

Closing this as there's been no follow up with info I could use to debug.

So something is causing your server to not have overworld level or is causing it to not exist at some point and this is where server wide data are saved as a standard by vanilla and mods. There's a ton of mods there in your modpack so you will need to figure out which mod that is and very likely report to them so that they fix. Or if you are able to recreate with sophisticated mods only please let me know steps I can use to recreate.

This seems to be happening to me as well, although in my case I'm assuming it's an incompatibility with Easy Anvils.