After checking the stereo track and comparing it to the 5.1, I definitely agree with your observation. The 5.1 is indeed faulty and not nearly as clear and full sounding as the stereo mix. Listening to the front speakers alone revealed some weird phase issues going on at the fronts, but inverting the phase of only one speaker didn't solve the issue. It might be that one channel is slightly delayed to the other, resulting in some weird sounding haas effect, something you can't resolve by inverting the phase by 180°.
This might be intentional to widen the stereo front for the 5.1 only to make the sound appear more three-dimensional to the listener. But I guess it's just another faulty disc with an authoring error. A real bummer since the set was quite expensive when I got it. I will write Nuclear Blast about it. I hope others will do the same.