1. You only have the ability to edit your posts for a few minutes, so be careful what you post.
2. I am not able to recreate your crash. I did exactly what you described... I created a sprite, gave it physics behavior, gave it a custom collision mask (a right triangle), and rotated the sprite beyond 45 degrees. It runs fine.
This means there is something else wrong with your .cap. You should send your .cap to Ashley so he can have a look at it. Otherwise it won't get fixed.