Thanks a lot. I will upload the .blend file then to the corresponding bug report. I used Blender 2.79. I hope that is not a problem as the other images are created with another version. I can mention the version in the filename.
Concerning the brown pixels I can check if I find a reason for them to get renderd. Maybe they can be avoided if the images is rendered in a higher resolution and afterwards scaled down using Gimp or another tool. I'll try that. But I can do it only on Monday or Tuesday next week.