Is the attic always colder than the ground floor?

Yes, doors were already closed. There are marginal deviations, from 0.2-0.4 degrees. I can explain that with the different floor coverings. I have not yet tried turning off [Außentemperatur Regelung]. I will try that. What has already been tried: mixing pump from level 1 to level 6 (maximum) -> no result.

I am a bit confused that you categorically state that the flow rate on the ground floor and basement has been throttled. Are you actually talking about the valves of the individual heating circuits for each room?
In brief, regards

Why a blanket statement? The heating engineer was here yesterday and turned every valve cap and adjusted the flow rates. And yes, every valve in the individual rooms...

By the way, I just checked the roof insulation... we have a U-value of 0.14... the energy saving ordinance requires 0.20. So it can't be due to poor insulation

That the floor temperature remains the same regardless of return and supply temperature is also strange. Possibly laminate with unfavorable insulating impact sound insulation?

No, unfortunately not either.

Floor coverings in the attic (where the problem exists) are carpet (fully glued, suitable for underfloor heating) and tiles (bathroom).
In the bathroom with tiles 21.2 degrees, in rooms with carpet 20.9.
No changes due to higher supply temperatures.

Does the heating also reach the set flow temperature?