Environment:
fmlloader-1.20.1-47.1.28.jar
Create 0.5.1d (Forge)
OPaC 0.19.0

Instructions: With only those two mods, claim an area, and in creative place a deployer into the claimed area.

Key start of error:
[17:32:35] [Server thread/ERROR] [ne.mi.ev.EventBus/EVENTBUS]: Exception caught during firing event: 'void net.minecraftforge.event.entity.EntityEvent$Size.setNewEyeHeight(float)' Index: 1 Listeners: 0: NORMAL 1: ASM: class com.simibubi.create.content.kinetics.deployer.DeployerFakePlayer deployerHasEyesOnHisFeet(Lnet/minecraftforge/event/entity/EntityEvent$Size;)V java.lang.NoSuchMethodError: 'void net.minecraftforge.event.entity.EntityEvent$Size.setNewEyeHeight(float)'

[17:32:35] [Server thread/ERROR] [minecraft/MinecraftServer]: Encountered an unexpected exception net.minecraft.ReportedException: Ticking block entity at net.minecraft.server.MinecraftServer.m_5703_(MinecraftServer.java:897) ~[client-1.20.1-20230612.114412-srg.jar%23132!/:?] {re:classloading,pl:accesstransformer:B,xf:fml:openpartiesandclaims:xaero_pac_minecraftserverclass}

My suspicion is "OPaC" sees the "fake player" introduced by the deployer and freaks out with an unhandled exception.

This bug was originally found in a modded game played in survival, and no other aspect of Create appears to have interfered with OPaC.

Did you test without OPAC? This doesn't look related.

You are correct, my apologies. I should have done more thorough testing.

Creators-of-Create/Create#5132 (comment)

This is just an FYI post in case someone encounters this here. The next comment, "nope, nothing to do with OPAC"