Attempting to use any kind of mob grinder or fake player to get player kill only loot will corrupt the server with an infinite loop of crashes until the offending mob/blocks are deleted in MCEdit:

---- Minecraft Crash Report ----
// I blame Mahtaran, author of Avatar Mod: Out of the Iceberg.

Time: 2018-10-29 09:03:00 MST
Description: Colliding entity with block

java.lang.IllegalArgumentException: Cannot get client player data for null player ID
at com.crowsofwar.gorecore.data.PlayerDataFetcherServer.fetch(PlayerDataFetcherServer.java:50)
at com.crowsofwar.gorecore.data.PlayerDataFetcherSided.fetch(PlayerDataFetcherSided.java:47)
at com.crowsofwar.gorecore.data.PlayerDataFetcher.fetch(PlayerDataFetcher.java:65)
at com.crowsofwar.gorecore.data.PlayerDataFetcher.fetch(PlayerDataFetcher.java:79)
at com.crowsofwar.avatar.common.data.BendingData.get(BendingData.java:83)
at com.crowsofwar.avatar.common.data.ctx.PlayerBender.getData(PlayerBender.java:57)
at com.crowsofwar.avatar.common.bending.fire.StatCtrlInfernoPunch.onInfernoPunch(StatCtrlInfernoPunch.java:133)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_131_StatCtrlInfernoPunch_onInfernoPunch_LivingAttackEvent.invoke(.dynamic)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90)
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182)
at net.minecraftforge.common.ForgeHooks.onLivingAttack(ForgeHooks.java:572)
at net.minecraft.entity.EntityLivingBase.attackEntityFrom(EntityLivingBase.java:850)
at net.minecraft.entity.monster.EntityMob.attackEntityFrom(EntityMob.java:70)
at net.minecraft.entity.monster.EntityZombie.attackEntityFrom(EntityZombie.java:233)
at com.rwtema.extrautils2.blocks.BlockSpike$SpikeType$3.hurtEntity(BlockSpike.java:241)
at com.rwtema.extrautils2.blocks.BlockSpike.onEntityCollidedWithBlock(BlockSpike.java:165)
at net.minecraft.entity.Entity.doBlockCollisions(Entity.java:1044)
at net.minecraft.entity.Entity.move(Entity.java:964)
at net.minecraft.entity.EntityLivingBase.travel(EntityLivingBase.java:1951)
at net.minecraft.entity.EntityLivingBase.onLivingUpdate(EntityLivingBase.java:2386)
at net.minecraft.entity.EntityLiving.onLivingUpdate(EntityLiving.java:577)
at net.minecraft.entity.monster.EntityMob.onLivingUpdate(EntityMob.java:45)
at net.minecraft.entity.monster.EntityZombie.onLivingUpdate(EntityZombie.java:223)
at net.minecraft.entity.EntityLivingBase.onUpdate(EntityLivingBase.java:2172)
at net.minecraft.entity.EntityLiving.onUpdate(EntityLiving.java:295)
at net.minecraft.entity.monster.EntityMob.onUpdate(EntityMob.java:50)
at net.minecraft.world.World.updateEntityWithOptionalForce(World.java:1993)
at net.minecraft.world.WorldServer.updateEntityWithOptionalForce(WorldServer.java:832)
at net.minecraft.world.World.updateEntity(World.java:1955)
at net.minecraft.world.World.redirect$updateEntity$zzo000(World.java:5117)
at net.minecraft.world.World.updateEntities(World.java:1759)
at net.minecraft.world.WorldServer.updateEntities(WorldServer.java:613)
at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:767)
at net.minecraft.server.dedicated.DedicatedServer.updateTimeLightAndEntities(DedicatedServer.java:397)
at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:668)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526)
at java.lang.Thread.run(Unknown Source)

Oof I'll try and fix that.

Ok this is fixed and will be in the next release