I've found that my graphic card drivers where not the latest available, so I upgraded them.

The problem went away if I do not use the textures for the walls. If I use only colors, 3D is working well.
But when using textures, the problem reappeared : from the left upper corner triagles are painted.
So it must be a graphic card driver problem, but because my card is rather an old one (matrox millennium G400), I don't think that new drivers will be made.

So I'm stucked on this one.