OK, I have figured out what was wrong with this DVD. The problem probably is that it contains 2 different video-systems: the main movie is PAL, but the extras are NTSC. I think this is not compliant with the DVD-Video specifications and that's why several DVD-players cannot play the disc correctly.

I was able to fix my copy, but now I really want to be sure that it is perfect. So my final question: the 'Concept art gallery' and the 'Game credits' are really silent? I think yes, but confirmation would be great!