Handlebar problem solved!
The support was great - they sent a new stem and handlebars.
I re-routed the brake lines and used sturdy downhill lines.
I greased the contact surfaces on the handlebars and stem really thickly with carbon grease.
Screws tightened crosswise in 0.2 Nm increments up to 4.4 Nm. Holds bombproof, tested for 530 km, even on potholes.
It was probably due to the paint on the clamping surfaces or the grip film on the handlebars, or it was a material defect. Because I did the same with the old stem and handlebars, also tried till 5Nm and had still the issue.
Just a theory, but now everything fits!
Many thanks for your help and thanks to velobuild!