[Monsters don't spawn at all]

I didn't find the cause of problem but fixed the effect, just installed mob spawn control that watches over number of mobs on the server and I guess it despawned them where ever they were spawned and now it's all right.

That might be some bug with dimensional doors because when I left the limbo I spawned 50 kilometres from home and I travelled all the way back eventually, they might get stuck somewhere on the way.

[Monsters don't spawn at all]

I recently moved to server pack Technic providing instead of Lan play and there's absolutely no night mobs spawning anywhere. Spawners work fine, nether mobs spawn fine.

I only added Thaumcraft over the build and no further changes.