Funny, just got my warning last week. iDrive was rather specific telling me the right rear light was out; checked and bulb is fine, put everything together and no warning for now.
I believe it's some sort of oxidation in the contact, on my X5 I just layer a thick high silver/tin on the contact area and no problem for years. Though I was stupid not checking for water under the batt while I was there (in case the sunroof drain is clogged).
Also I don't understang why BMW can't make the contact area gold plated, it shouldn't cost that much more to do so especially if you can pick up a few point on JD Powers ratings in the process.