Bumping this thread because this just happened to my M340i today. Ambient outdoor temperatures are very cool (50 deg. F), and others have said that this is not a cooling problem with the amp. Probably some sort of safety cutoff after high usage. I did have some bass-heavy music going at about 40% to 50% volume for a while. Found a thread on the 2-series forum, but not much else. It does seem like a known issue with HK amps in other BMW models.
Anyone else with a G20 experience this? FWIW, a hard reset of iDrive did nothing. Had to shut off the car completely (let all electronics go into sleep mode), wait 5 minutes, and then restart. Worked properly after that.
https://www.2addicts.com/forums/show....php?t=1288100