New info: When I'm far away from that wall everything looks ok, when I come closer, the problem apears and when I am very close it disapears again ! Very strange. And it only apears on that kind of wall, that you see affected in the photo. On walls made of bricks or stone everything's ok. Those rainbow bands are not stationary, when I move the mouse the bands are flickering and moving. It really looks horrible but at least they aren't everywhere. However something doesn't work corectly.