ok.. first of all this is a BMW- website .. ergo the BMW- Syndikat. But as we are not a rude bunch I am sure someone with knowledge can help you...
I havent really looked at a diesel motor but it might be just what it says the positioning sensor.
There are a lot of variables...
Could be dirty, the adjustment screw is loose ans it moved, could be your computer is all screwed up and you need a new one.
Then there is the obvious what is a BMW engine doing in a Opel??
But I dont know.. can someone else assist??