As a minor point, I did experience something similar in my world with the DW20 pack. The server was relatively low-spec. What was happening is that the mobs were simply not spawning naturally. Stuff wasn't despawning (my chicken farm was still fine), they were still spawning for spawners (vanilla and soul shards), and were still spawning in other worlds (nether and twilight forest). The issue would be fixed by a restart, and then would occur again after a day or so. I never really got to the bottom of it, and the world was deleted before I did any major bug hunting on it. It did make minium stones much more difficult to find, though.