Hi Eugenio thanks for posting the file. So I couldn't see anything wrong with your model so I took it into Rhino to check it in there too and then I was able to see the problem is with this particular edge here:



Some other underneath views of it:





So it's something like this where there are 4 surfaces all touching along that one single edge:



So that's what's called a "non-manifold" edge, and it can cause complications for determining the proper inside and outside of a solid volume. It's kind of like a zone of zero wall thickness and if something tries to intersect that edge it won't be clear to it which side it's happening on. It's similar to the problems caused if a surface intersects itself, in this case it's like the "skin" of the solid is intersecting itself.

That looks like the only spot with that problem.

- Michael