By default, residents do not receive damage from zombies, the way to replicate the bug:

1. Disable creature protection in private;

2. Let zombies near a resident;

3. The server crashes.

crash-2025-09-14_09.46.06-server.txt

crash-2025-09-14_09.47.07-server.txt

at net.minecraft.world.entity.Mob.setSpawnCancelled
at net.minecraftforge.event.entity.living.MobSpawnEvent$FinalizeSpawn.setSpawnCancelled
at xaero.pac.common.event.CommonEventsForge.onMobCheckSpawn
at xaero.pac.common.event.__CommonEventsForge_onMobCheckSpawn_FinalizeSpawn.invoke(.dynamic) ~[open-parties-and-claims-forge-1.20.1-0.25.3.jar%23254!/:?]
opac is literally cancellng an event wrong
and yes it does touch spawning

This actually looks more like a bug in Forge because I am in fact using the event according to the documentation.
It's very easy to work around on my end though.

Fixed in 0.25.6.