I found 22 degenerate polygons in the model and deleted them. They were shown  in Lightwave as non-planar triangles. They appear on the display as lines.

I am getting some bad shading in the render due to the small triangles along the tops of the walls. I would merge the triangles and split it differently but your modeller may not allow that.

I am attaching the modified obj file. I have lost the uv mapping if there was any.