After playing with my excessively large mod list in a world with no issues for a few weeks, my world has finally come grinding to a stand still, crashing upon load or shortly after load. FoamFix has intercepted the crash and reported that the error was caused by OpenTerrainGenerator, but the issue is related to biomes, so I'll post it here first.

Crash report attached.
crash-2018-06-27_15.46.08-server.txt

The error log is not descriptive enough to know exactly what happened. The only way we can identify and fix the issue is if you're able to identify which other mod is causing the problem with OTG.

Ah that makes it tricky then. I've seen this error occur sometimes without mods I believe. Will keep testing though!

Okay I've found the mod - Armoury Expansion
https://minecraft.curseforge.com/projects/armory-expansion

Removing the mod solves the issues and no more crashes have been encountered.