We did manage to find something that was broken. Imagine setBlock not actually ending up setting the Block with a 0.0001% chance, whenever it is in a chunk that has its neighbors not generated yet. Yes that kind of bullshit MIGHT have been the cause. XD
It might be fixed in the Dev Version now, but i doubt you could replicate it anyways, since it not only needs a Seed we dont know yet, but also needs the player to approach the crashing location at a certain angle.
This Version should probably be working for you. ^^
(note 1: The scary sounding Description is only there to make sure people don’t use that Version lightly.)
(note 2: if you use this Version more often, make sure your Browser didn’t cache the Download)