It has disagreed (as the rest of your post suggest) so you cannot be forced to use coupons in sequential order. However, and this is one heck of a huge 'however', the airline can then charge you the difference between what the price would have been had you booked your itinerary as flown in the first place. So, under that ruling, they cannot refuse you boarding if you turn up for the second leg but they can make you pay mightily for the priviledge.

As Jouy31 stated, in any event, this only applies to German courts.