Thank you for your response
I already have a hollow box. As I described, the only difference between the working version and the non working version, is a model added in the script (ent_create) and a model added in WED.
That's the only difference! This is driving me crazy...
For now, I can continue working in my project... But it's hard, because eventually i'll want to start using the level editor to make.. well.. levels! Making a whole level with ent_create just won't happen.
I'm starting to think the problem is with my installation..