[1.12.2] permanent java.lang.IllegalArgumentException

[Lefroel]

Hi, I am using Forge 1.12.2 - 14.23.5.2854 with IntelliJ IDEA 2020.2 (Community Edition). And every time I press runClient everything is going well, but at the end of everything I get the java.lang.IllegalArgumentException even with absolutely clear new example mod. My minecraft runs properly and it even sees my main mod file, but it seems like it can't see my resources package. Everything I put there including mcmod.info and some textures and jsons are not loaded. I'm only starting here, so I can't even imagine where the problem could be.

For checking I used the very standard example mod, so I see no point in putting any code here.