Hi, I had the same or similar fault on my A6 40 s line 21 plate. I was driving on a busy dual carriageway with no hard shoulder when the warning came up and the engine just died, no power at all, no throttle, I was just drifting with the vehicle slowing down. I had to put my hazard lights on and stop in the lane. Pretty scary with cars approaching fast from behind. I turned everything off, slightly opened the door and closed it to reset everything and luckily the car started up and I was able to drive off. Potentially that could have been disastrous.
I took the vehicle to the dealership and they told me that there had been a sporadic fault with the "can bus", they cleared the fault and asked me to monitor. Although not sure how I do that. They had no idea what had happened or why, just cleared the fault and sent me on my way.
I have told them that's not good enough and what happened Could have resulted in a serious accident and that I'm no longer confident with the safety of the vehicle. I'm currently in discussions with them to either exchange or return the vehicle.
I'd be interested to know if you know of any other incidents of this fault as Audi don't seem to be that bothered that this fault could have resulted in a serious or fatal accident.