Alright, so first and foremost- I apologize if this is in the wrong board!
Now, onto the solution:
If your game is crashing (I use FTB Ultimate mainly, but this should work with other packs), it could be because of a very simple problem- the launcher cannot access files it needs to generate your world (i.e. the chunks), so it gives you the error.
The easiest way to fix this is as follows:
Right click your launcher; a white menu box should pop up. Mouse over the "Run as administrator" option, and click that. this allows the launcher to access the files it needs to to generate your world.
I hope this helps anyone experiencing this error out! I had to figure this out by myself, as this error is usually caused by lack of memory etc. Anyways- it was weird.
(Again, I apologize if this is in the wrong section, I'm just trying to help!)