I want to let you know the findings.

The output flange nut came loose as suspected by some of you. The most likely reason for this was that the driveshaft and the output flange were connected wrong to the flex disc. The purpose of the flexdisc is to decouple transmission from the drive shaft from a rotation/torque point. Therefore the coupling between both is made with the rubber of the flex disc. On my car the output flange was bolted directly to the driveshaft through the flex disc. In other words the flex disc was only used as a spacer but not as a decoupler. So 3 screws were used only to fill the remaining holes in the disc :-) I assume that vibration and shear forces made the nut of the output flange came loose.

You may want to check your car if the previous owner/service messed it up, too.

Thank you for your quick support !