well Mr Kukukachu, that's a pain. From the start, Ive been very skeptical of the whole locking thing on these, and never considered using it, partly because I never use a bike lock or anything with a key on a bike trip for fear of losing the key, but also of course because to me it really doesnt have a proper function anyway (well, maybe more a bit now with the locking lid versions with the magnet lid models nowadays).
I take it that you locked it for storage or something? Ive actually never locked mine, and have the keys somewhere.
this is a real far shot, but there might be batches that use the same key cut, although being German, they probably did go the "every lock has a diff key route".
I recall a number being stamped somewhere for replacing keys, is it on the mount or the keys themselves? I'm sure Ortlieb will get back to you, although I wonder if the cost for a key is not perhaps half of just getting a new mount kit, and you could drill out the lock and stick in a wooden dowel--I have seen a photo on here probably of someone doing just that, drill out the lock and use the dowel as leverage to move the spring mechanism for the actual clip thing that holds the bag on.
do some searches for the "drill out" method, if you never use the key, this would be the easiest and no money spent.
you know, it might be Doug64 who has done this, try looking it up.