When you have other mods installed which also change the character_inventory_slots_bonus, this mod breaks them. For example RPG System can't set its level based inventory size because this mod resets the value. The issue is in the enable_modifiers and disable_modifiers functions. It should probably keep track of how much bonus a player was given previously and add/subtract from that.

Good point on that! Thank you for the research you've done so far, I will fix that

Interesting thing tho - other modifications are mod-scope only, and not affecting other mod modifications