I am getting stability control and suspension control failure errors as of saturday. I put in the braile 21lb light weight battery about 2-3 months ago. It worked fine for a month, then I noticed the comfort access would not work when trying to open the door. It always works when exiting to lock the door. A few days later I noticed the yellow, low battery light coming on. Now it comes on many times and the comfort access will always fail when trying to unlock.
I think as far as the OP is concerned, the dealer may have been right, and that all kinds of weird errors can pop up and cause warning lights from a weak battey. I have seen a lamda voltage error in the 02 sensor, and just got a 5D97 - Medium or High Voltage Short Circuit of voltage supply FR or the valve on ground registered during the cyclical valve relay test.
I'm thinking the battery is not suitable for cars that draw current all the time with Comfort Access (there has been much discussion of these types of prolems on the M3 board. I do mainly very small trips around town, and the car is sometimes driven only once or twice per week.
Going to get a battery tender, charge the OEM back up and put it back in, to se if all the errors go away.