Monsters stopped spawning

I created a server with a few mods on it, nothing that should change the way vanilla mobs spawn, and at first everything worked perfectly fine. As we started exploring the world more, hostile monsters just stopped spawning in the overworld. Animals and a few modded mobs spawn, but regular hostile monsters just wont. They also spawn normally in the nether, so it's just something wrong with the overworld.

The version is 1.12.2.

millenaireserversmp.aternos.me

Also I have biomesoplenty installed, but I forgot to set the world type. That wouldn't cause the problem, would it?

I'll try to see what happens on a vanilla server. It took quite a bit of time for the problem to start occurring, so it might be rather hard to recreate.