Ok, at some point I have apparently downloaded custom stairs that my game doesn't agree with. When I'm in build mode I click on the stairs icon and.... crash... every time.
I thought it might be just a coincidence that it crashed when I clicked the icon but I changed houses and got the same thing (several times).
My question is: Since looking through my massive downloads folder trying to find anything with stairs in the name is proving to be useless, is there a better way to find out the problem that's causing the crashes?