Insufficient screed insulation

Impact sound improvement measure VM = 24-28 dB is the insulation. Apartment separating ceilings 0.75 m²K/W -> 28 mm Rolljet 30-2 - according to the flyer at least meets the minimum requirement of the energy saving regulation.

Because of a larger volume flow

Without further measures, a larger cross-section leads to a lower speed or lower pressure loss (whether the flow rate then changes depends ...). The (nominal) volume flow is actually determined by the heating load and delta T. I would like to learn more about why exactly you changed something. --- And whether that is really necessary/sensible. In my opinion, the question is justified due to the consequential problems with the floor structure.

Now there are two things: we have decided on larger underfloor heating pipes

Why do you also delve so deeply into the subject of house construction ..... What solution does your architect have regarding screed? Rhineland greetings

Floating screed - I really need to ask about that again. Why... good question actually heard too much on forums.

Screed is always floating - otherwise you will have real sound and crack problems

So I haven't quite understood where your problem is now, but we have a total of 16 cm floor construction throughout the entire house. That means after the concrete (floor slab or ceiling) we have 5-6 cm insulation, then 3 cm impact sound insulation, 1.7 cm underfloor heating pipes, and then another 4-5 cm screed on top. In unheated basement rooms, we have 10 cm insulation and then 4-5 cm screed on top. Insulation was also installed under the floor slab, but I don't know offhand how thick it was. Why do you have different heights on the various floors? And why do you have such thick insulation on the floor?