Seems to be the exact same issue as #101, but on the latest version of Citadel. Happening consistently with this pack on all platforms, and both client and server-side.

I spent like twelve hours straight going through it with a debugger trying to find why it was doing that, so if you need any more information, please let me know.

Citadel: citadel-2.6.0-1.20.1
TerraBlender: Disabled

Modpack: Epoch Arcanus

level.dat.txt (an extension that GitHub will accept)

It looks like it might be an incompatibility with Luminous: Nether, though how an MCreator mod could do that is beyond me.

Here's a stringified version of the level.dat I managed to get the server jar to generate: level.txt.gz

(That was so nostalgic btw, having to modify the server jar to bypass the limit. Amazing to see how old-fashioned Minecraft mods were made, down to deleting META-INF to get rid of the checksums and signing.)