After removing a worldgen mod from a existing world, I get this spammed in the client console. I assume thats related to the blocks that don't exist anymore.

http://hastebin.com/rupebidusu.avrasm

Running on forge-1.9-12.16.0.1865 with MW 2.4.15

could you update to a later version and see what the console spams then? (it should spittout a more meaningfull error.)