crash log: https://paste.dimdev.org/domezoboxe.mccrash
Minecraft version: 1.12.2
modpack: stoneblock survival 1.0.32 (modified) (ffenchantments added manually)
mod version: MobGrindingUtils-0.3.13.jar
conflicting mod: ffenchants-1.12.2-1.1.3.jar (by flashfyre)

when a minecraft:tile.mob_grinding_utils.saw (mob masher) encounters a mod with an enchantment named "Cursed Body", the server crashes. and takes at least 5-10 restarts of the server for it to successfully restart, usually.

Hello,
This is more likely whatever mod adds that effect is trying to add it to the masher. They should be checking in their code if the player is a Fake player. If they don't then stuff like this can happen.
If you could also report to the other mod that this is happening they may be able to fix the issue.
Cheers,
Me.
*Edit - I have spoken to FlαshFyrҽ and he has found the issue and helped out by adding a fix. :)
When he updates FFEnchantments it should play nice with MGU.