Issue as linked here
MysticMods/MysticalWorld#226
Related Commit
MysticMods/MysticalWorld@e4838cb

Current Behavior

mysticalworld hellsprouts overspawn in the nether and take down server
Spawn rates for nether seem to be much higher than 1.6.0, possibly related?

Possible Solution(s)

1. Update to mysticalworld-1.12.2-1.11.0, mysticallib-1.12.2-1.13.0, and Roots-1.12.2-3.1.4 [did not fix immediately]
2. Edit file to mysticalworld.cfg (it may have been mentioned that lava cats were also overspawning) [did not test]

        # Enable the spawning of Hell Sprouts
        B:spawnHellSprout=false

        # Enable the spawning of Lava Cats
        B:spawnLavaCat=false

3. Updated to mysticalworld-1.12.2-1.11.0, mysticallib-1.12.2-1.13.0, and Roots-1.12.2-3.1.4; Deleted /world/DIM-1; deleted /world/config/netherex/ [shotgun approach seems to have worked]
4. Tune nether spawn rates back to 1.6.0 levels

Steps to Reproduce

Upgraded server from 1.6.0 to 2.0.0
-downloaded server
-copied world folder
-started server
-Visited nether

Did not test fresh install