Light behavior is not correct Although the normal vectors of the faces looks correct, the way the object is illuminated by the light source is wrong:



As you can see the light source is very close to the left side of the cube, yet it's not fully visible, but the front face is fully visible, and if I look the right side of the cube, I see this:



Which is also...weird. 

What am I doing wrong? What and where should I check? If it matters, I generate an .obj file which I import later into Blender.

Here is the .blend file I'm using:
The light behaviour is working as it should. Your mesh isn't...

You have been playing with the normals of your object, and they are all wrong. They should be perpendicular to each face, but instead they are tangents to the faces.
This was probably something you did in python since there's no interface to do it in the UI.

For a quick fix:

Properties Editor> Object Data> Geometry Data> Clear Custom Split Normals Data

I advise you to review your python code to avoid future situations like this one.