I had a similar problem on my ML320. It turned out to be a fault brake light switch. The dealer replaced it under warranty, so I don't know much more than that. "A known problem on the ML's" I was told by the dealer.

When the brake light switch went out the sypmtoms were the same as you described. When I started the vehicle, the BAS/ESP lights turned off. As soon as I stepped on the brake pedal they came back on. My brake lights did not light up. Also, it would not shift out of park, and stepping on the brake pedal would not cancel the cruise control any more.

That's about all I know. Sorry if it doesn't help in your case.

George