Installed mods:
Forge 43.2.17
Blueprint 6.1.2
Neapolitan 4.0.2

Using the following config causes a worldgen crash when trying to place chimpanzees:

[mobs.chimpanzee]
	#The percentage chance for a Banana Plant to generate with a group of Chimpanzees
	"Chimpanzee group chance" = 0.25
	#The maximum amount of Chimpanzees that can spawn with a Banana Plant
	"Chimpanzee maximum group size" = 0
	#The minimum attempts for Chimpanzees to spawn with a Banana Plant
	"Chimpanzee minimum spawn attempts" = 0
	#The maximum attempts for Chimpanzees to spawn with a Banana Plant
	"Chimpanzee maximum spawn attempts" = 0

I don't have a crash log because the game hangs when this happens, but i do have the game's output log which shows the exception: https://hastebin.com/share/eyupomipec.php

I can confirm the server i help run is getting the same crash with an identical log:
Log

and also can confirm that it is solved by raising max spawn attempts to at least 1

Output log no longer available...

Ah, sorry about that. Here's a new one: https://pastebin.com/MMtvtjeT