I have same problem . . .
with my 2007 335i coupe. Sometimes when I try to press on the brake pedal in order to start the car, the pedal resistance is as hard as a rock. At first I was not able to start the car but then I would just apply alot of force from my foot to the brake pedal and that would allow the car to start. After the car starts the brake pedal resistance goes back to normal. Weird.