My advice would be:

(a) make it as simple for them as possible by labeling all your documents with the PNR and description (e.g. XXXX-original-booking.pdf, XXXX-reroute.pdf and so on...)
(b) make sure you really include everything (boarding passes and passenger recipes, cancellation notifications, etc.). I had some trouble using just the confirmation emails instead of the passenger receipts because they don't show details such as invol. reroute endorsements.
(c) write to the German-speaking support to bypass the outsourced agents
(d) finally, be persistent if you get a generic denial

Good luck!