kuhni74
ah, the number is in fact 2.401km² (in Anglo-Saxon notation) which reads 2,401km² (in German notation), so this should be right.
The guys from historic.place told me, however, that the correct tags should be "boundary=protected_area" with "protect_class=22", according to  https://wiki.openstreetmap.org/wiki/DE:Key:protect_class