Hi I have this problem while starting the game:
https://pastebin.com/DLVJAKze

same problem here
Forge: 1.15.2-31.1.18
apth: Apotheosis-1.15.2-3.0.6
placebo: Placebo-1.15.2-3.0.2

Seems to be your item
ItemHellBookshelf.func_150895_a(ItemHellBookshelf.java:47)

Yeah but I would like to know a way to solve this issue, I mean I know that item is the cause but I don't know what to do

This is being caused by another mod invoking getSubItems earlier than it should be, but the trace doesn't provide what mod. The debug.log file might

im thinking it could be flux

But I always had these two mods since the beginning of this modpack! How could that be possible?

Loading is multithreaded, when it's happening (and it may not be that mod) could be a result of load order weirdness

Are you kidding me, mr mod? SDUBZ, Shadow of Fire, you guys were right... It was because of Flux, that's ridiculous... Anyway, good for me now :D

At least the mod still works but only if I downgrade to the previous version of Flux, because I saw that I'm not the only who have this issue