Finally I got a step further to solve my vertex welding problem in my voxel terrain thing. The artifacts you see are the chunk borders and an annoying rounding error I did concerning positive and negative coordiantes. I implemented a perlin noise generator to build random terrain.
I think I will kick the automatic normal calculation and calculate them myself to avoid the black borders and sharp edges.
Keep in mind that the terrain you see there is huge. It doesn't look like it, because the perlin frequency is too high.