If you change the level on server.properties to a symlink of a folder, it tries to load the world, but instead crashes immediately. On vanilla world loads as intended. Tested using CWG33 and CC 903 and a seperate Vanilla 1.12.2 server.

... why are you using a snapshot and a forge mod together...

Cubic Chunks only works with Vanilla 1.10, 1.11 and 1.12, why are you using a snapshot?

He tested using plain non cubic chunks vanilla with snapshot, version is irrelevant here. This is just to confirm this works in vanilla.