I made a server with your mod in 1.12.2 with forge version 1.12.2-14.23.5.2847.
It loaded all the other mods but crashed:

An error occurred trying to load an EventBusSubscriber com.jozufozu.exnihiloomnia.common.items.ExNihiloItems for modid exnihiloomnia
java.lang.NoClassDefFoundError: net/minecraft/client/util/ITooltipFlag

It seems that you are using a client method in the server.