First, I have gone crazy for a year trying to find the exact scenario that I've had, but haven't really found it.
Mine started about about 15 months ago. Had about 107K on my F25 (n55). On a long trip, returning from about 3 hours from my home, threw a "Drivetrain Error" on the highway. No loss of power felt. Drove about 45 minutes to get off the highway, then stopped, restarted the car and the error cleared. It threw once or twice more, then I got the Schwaben ECU reader. Had the 120308 error. Cleared it, but it kept coming back (with the chime and the "Drivetrain error"). Never lost power when it came up.
Replaced the pneumatic wastegate actuator (fairly easy job, but angle is a PITA). No luck. Replaced all vacuum hoses, no luck.
At 108.5K, engine failure (unrelated to 120308 and a whole other story). New long block engine, which added a new turbo and a new metal charge pipe. Got it back and it was still throwing the 120308. I knew since the CP, turbo and block were new, it had to be a part that wasn't replaced or something broken/misconnected during the engine replacement. Replaced some plastic vacuum piping that looked suspect or had broken connectors (others had mentioned that those parts fixed their 120308, or I felt that they needed replacing at 109K+ miles).
Finally, I replaced the diverter valve, as I had suspected it might be a cause, but was reluctant because it is beyond a PITA to remove and replace. I was never so happy to see a broken part as I was when I finally got the DV out of the turbo.
Attachment 2502664
Attachment 2502665
The rubber membrane was ripped about 1/4 to 1/3 of the way around. It was bad enough that it makes me wonder why it didn't throw 120308 or other errors more often.
I had ordered the GFB (Go Fast Bits) DV upgrade and was greatly helped by this video
https://www.youtube.com/watch?v=dQYhu13pLu4. Install took about 4 hours, slowed by the fact that my broken DV was mounted with Torx screws instead of Allen screws, which I guess happened when the block/turbo was replaced.
I took it for a drive long enough to get it to throw the error and tried the scenario that made it throw the most: In motion at under 3K, push peddle slightly to accelerate (think: don't downshift, but accelerate, as if merging onto the highway or slowly passing a car when making a lane change). Even tried hard acceleration from a stop or single/double gear downshift.
NO ERROR.
The GFB DV gives better peddle response and it now idles and runs generally smother than it did. The part was $150. I would think an indy shop would install it for about $500-600 in labor. BMW would probably be $1K or more and you'd get the original, weak OEM BMW DV.
I hope your problem was solved, or that this might be of use for you or others.