Im am not sure if this is a problem caused by better caves, but there is no mention of any mod causing it and the last thing to see before it crashes is the carver initialising.

It seems to have something to do with mob spawners generating.

Caused by: java.lang.IllegalArgumentException
	at net.minecraft.util.WeightedRandom.func_76273_a(SourceFile:20) ~[?:?]
	at net.minecraft.util.WeightedRandom.func_76271_a(SourceFile:39) ~[?:?]
	at net.minecraft.world.spawner.WorldEntitySpawner.func_77191_a(WorldEntitySpawner.java:335) ~[?:?]

Full crash report

Forge version: latest 1.16.1
Mod version: 1.16.1-1.0

Was the fault of another mod