Could it be that it's something else that's causing the crashing? Maybe we are barking up the wrong tree. I never use custom terrains myself, and do not recall ever clicking on it accidentally. And why would opening the terrain catalog cause something to happen to the lot. That doesn't make sense. Maybe some modder with a good understanding of the game mechanics could explain that one.
Once you open the terrain file, every paint in it becomes part of your lot. Move the lot to a vanilla hood with no custom terrain paints and BOOM! It's well-documented that this happens! As I said, I've lots made around the same time in the same hood which load with no problems whatsoever, while others crash. The only possible difference that I can think of is the addition of a custom terrain paint!