I could only conclude (from looking from the 'top' position, that I was exactly on a lie that I saw in the line preview mode. In the walkthrough, I ended up in the cellar of the building and then a couple of times only the hole from the stairs went black for one frame. It is strange. I editeted out most of this artefacts and multiplied the animation with a VectorStyle flash-version of the same animation. Layering video and rendered stills makes it a nice piece of video after all. But I hope Eovia will spend some time with the NPR to solve this things. I do not have enough time te experiment right now. Thanks for reading and giving it some thought folks!