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 ...

Hello,

When trying to make walls, the 3D view is following like expected.
But when moving around in 3D other lines and triangles from the corners of the 3D view are put on screen. Sometimes they cover completely the design.

Other problem is when using a texture for the walls. Then I have from ...