Added Hatchery after I had gen'd some of the world (to the point of killing the dragon) and started getting this crash after trying to generate new land, even in the End.

java.lang.IllegalArgumentException
at net.minecraft.util.WeightedRandom.func_76273_a(SourceFile:18)
at net.minecraft.util.WeightedRandom.func_76271_a(SourceFile:37)
at net.minecraft.world.WorldServer.func_175734_a(WorldServer.java:241)
at net.minecraft.world.WorldEntitySpawner.func_77192_a(WorldEntitySpawner.java:126)
at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:198)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:698)
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:387)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:613)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471)
at java.lang.Thread.run(Unknown Source)

-- Head --
Thread: Server thread
Stacktrace:
at net.minecraft.util.WeightedRandom.func_76273_a(SourceFile:18)
at net.minecraft.util.WeightedRandom.func_76271_a(SourceFile:37)
at net.minecraft.world.WorldServer.func_175734_a(WorldServer.java:241)
at net.minecraft.world.WorldEntitySpawner.func_77192_a(WorldEntitySpawner.java:126)
at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:198)

Further debugged by removing Hatchery and going to the same areas, no server crash. Can also prevent the crash by booting server, doing /gamerule doMobSpawning false then moving to a known safe area and setting it back to true. I'm hoping that fix will help you narrow down the issue. Also mentioned this as Beyond because I built off that pack so you know which other mods there are.

Did you happen to make the new Lucky egg machine?
or use lucky eggs?

This is happening on world gen.. when spawning entities.. Roosters are the only thing that spawn in with hatchery, and they aren't anything special. just a simple mob using vanilla spawn mechanics.

No, I did not. As I mentioned, the doMobSpawning gamerule being disabled prevented the crash.

Looking back at the log file, this is with the 0.3.11 version. But I was unable to find any information related to this anywhere, so it's possibly still relevant. I'll try the latest version tomorrow.

Unfortunately that crash isn't specific at all, its not your fault. My guess is the ModPack may be messing with mob spawns with another mod, and its interfering with hatchery. I cant say this is a mod issue, because it could be a pack issue. I would need other reports to really narrow it down. I appreciate ya reporting, and if this is a mod issue I will be glad to fix.

Is the Rooster config spawn weight/percentage set to 0?

#115

It was, yes. Wasn't aware that disabling them would cause this, I just wanted to save a little on system resources. Maybe some kind of check for the config file that sees the weight chance as 0 and just disables the entire rooster spawning system instead of having to add a new config option and/or throw errors when its set to 0?

I'll set it up to not do 0, but using a mod like https://minecraft.curseforge.com/projects/in-control gives you complete control over entity spawning.