Well cautiously saying, I guess it's turning out just fine... you were right to keep it cool.
Apparently the fuel-return-tube came loose from the engine, probable wasn't 100% properly attached and because of the pressure on the tube it snapped. The returning fuel leaked on the motor underneath the cover in a dent and because of the engine temperature, it started to "boil/burn up/smoke"
My dealer was very supporting. They fixed, checked, double-checked, steamcleaned the engine, cleaned the body again and presented it again on the delivery stand.
Then when I got home this evening, I finaly had the occasion to show it to my lovely wife.
I'll try to get the BMW Joy back soon, feeling the (for now) carefully hidden power underneath the pedal will bring it back real soon I'm sure... the sound and feel of the engine is in no way comparable with my old 335d.
I will also post some camera-pictures and some thoughts in the upcoming days...
Thanks for the support everyone!