Ok I've sent some models.

I've recognized something else by now:
When I create a new model with a single quad (4vertices two faces) this simple model works right with ent_clone. However when I add two faces to the existing 4 vertices with build face, that the model is also visible from the opposite direction, it crashes with ent_clone.
So a two faces quad that has only a surface on one side works while a four faces quad with a surface on both sides crashes.