Is the attic always colder than the ground floor?

An zu schlechter Dämmung kann es also nicht liegen

It's not about good or bad... simply put, double the amount of exterior surface, whether wall or roof, means twice as much loss...

The upper floor should also be able to be heated to > 21 °C! Most people have a bathroom or children's room on the upper floor and there it is certainly often more than 21 °C.

trotz aller Maßnahmen niemals über 21,2 Grad Raumtemperatur kommen

I consider this a defect. Have you reported this formally yet? What else could be done: - Have the heating load calculation and dimensioning of the heating system checked by an independent expert (is the paper specification even correct?), if okay, then: - Measure the flow and(!) return temperature and(!) flow rate for each room to determine the heat input (full load case). --> Is it due to insufficient heat input or heat losses? Keep us updated, it's an interesting topic .

Do you actually always use the same thermometer? I have noticed large differences with both digital and analog ones. And is the thermometer located in the middle of the room or is it possibly hanging on the exterior wall?

From my point of view, the problem must be in the supply temperature. You say the flow rate is 2 liters/min, but increasing the supply temperature does not change the surface temperature of the floor. THAT cannot be, completely independent of the insulation.

So check the thermometer reading.

Your description actually does not make any coherent sense. There must be a mistake somewhere. A couple more questions: How long did you keep the changed settings and is the same screed poured throughout the entire house (type and thickness) or do you have insulation under the floor slab or above it?

One could really only advise to try again as a test: 1. note all settings 2. lock at the bottom 3. open at the top, building circuit pump full throttle 4. watch what happens (at least 24h) 5. increase flow temperature, repeat from 4.

Admittedly, it is a bit unconventional and it gets a bit cooler at the bottom, but it should provide clues

If all this is too stupid/complicated for you, then you need to get your contract partner or an external professional to continue troubleshooting...