Problem Hostile mobs not spawning Horizons modpack

[Thronk]

We're running a Horizons server with MCPC+ and all was fine till I updated to the latest Cauldron, then we noticed hostile mobs sometimes not spawning. I reverted back to just Forge and they're still not spawning. Changing the difficulty sometimes brings them back, but only temporary.

I've searched the internet for other's with the same problems, tried all what they've suggested to even install the Mob Control Mod and I still can't get the hostile mobs to spawn.

The gamerule doMobSpawning is set to true. No unusual error logs or high spawn rates of other creatures.
I think when updating to Cauldron (version 89) it set something in the configs somewhere. I looked at the level.dat file and everything is ok.

It's been over a week I've been trying to figure this out, hope someone can help me out.

Edit: Mobs are indeed spawning in every other dimension, just not overworld.

 

[Wagon153]

I was on a server where this was a problem. We fixed it by pretty much removing Farlanders and disabling Ars Magica Hecates.

 

[Thronk]

Update: I did notice hostile mobs spawning in vanila biomes only.

I was on a server where this was a problem. We fixed it by pretty much removing Farlanders and disabling Ars Magica Hecates.

Thanks, I'll try that.

 

[Thronk]

Disabling Farlanders and Hecates didn't resolve the problem.
It has something to do with biomes, hostile mobs were sometimes only spawning in vanilla biomes, and sometimes not at all. I tried setting "Add biomes to Default World"=false in Biomes o Plenty config, and still getting the same.
All my tests are done on Forge only, MCPC+ isn't enabled.
I'm still trying to resolve this problem.

After seeing mobs only spawning in vanilla only biomes, I traveled to the border biome "Mesa" and set the Mob Spawn Control mod to spawn FireBats in the Mesa biome for the hell of it, then mobs all mobs started to spawn in that Mesa and other biomes. Though, I can bet that this is only temporary as with the other attempts.

 

[Thronk]

I still need to periodically change the difficulty to enable hostile mob spawning. At this rate, since I cannot find the cause, my only solution is to schedule the toggling of the difficulty command automatically.