I have the same problem with all 8 of my locks, they will all happily lock and unlock with the plastic bung.
I reported this to my dealer back in January and it they got on to BMW. It took until the end of April for BMW to come up wit the "solution". Except that their idea of a solution was not to fix the locks but to replace the bung with a slightly shorter one! So now you can only steal all my panniers if you bring your own bit of plactic instead of using the one that is already on the bike. I was not impressed!
After some experiments with another new lock, the dealer claimed to have fixed the problem and said it was down to too much greaase in the locks stopping the plates moving freely inside. Sure, and I'm Abraham Lincoln! They were supposed to have fixed my locks but when I got the panniers back sure enough they weren't fixed.
The problem is obviosuly widespread enough for BMW to make a new bung for the lock. Presumably this also means that the locks are not "faulty", just badly designed. I think it depends on your ignition key and how similar it is to the shape of the bung.
There is an address for Customer Services in the back of the warranty book. I wrote them a snotty letter this week, still waiting for a reply.
I sugget everyone else with the locks problem writes them a snotty letter too and maybe we will get some action!