What happened?

Minecraft 1.21.1
NeoForge 21.1.208

After installing the mods listed below, worlds will not load, immediately kicking the user with "Invalid Player Data".

Yes, I'm aware that the suspects are Unify and Oreganized, but it can't be reproduced without the others.

Logs: https://gist.github.com/CraftyZombie/2744fda74f3c830b1d68927657c5881b

Relevant log output

[Server thread/ERROR] [minecraft/ServerConfigurationPacketListenerImpl]: Couldn't place player in world
net.neoforged.fml.ModLoadingException: Loading errors encountered:
	- Oreganized (oreganized) encountered an error while dispatching the net.neoforged.neoforge.event.BuildCreativeModeTabContentsEvent event
	  java.lang.IllegalArgumentException: Itemstack 1 unify:lead_block already exists in the tab's list

Report it to oreganized please.

Yeah, that confused me as well. However, the bug cannot be reproduced without Mekanism.

None of that points to Mekanism that I can see