Gas pedal and parking brake of the heater: Explanation correct?

That has to do with the heating curve. I don't know exactly how your manufacturer defines the desired temperature. Please provide more precise data about your system. Then you can be helped better.

So we have:

Vaillant Arotherm VWL 55/5 with Unitower 58/5. The central control element is called "SensoComfort"

In the instruction manual for Sensocomfort, some terms such as flow temperature and heating curve are explained/mentioned. Please check with which flow temperature the system operates.

I have taken some data:

Water pressure: 1.5 bar
Adaptive heating curve: deactivated
Hybrid manager: bivalence point
Bivalence point heating: 0°C
Bivalence point hot water: -7°C
Emergency operation temp.: 25°C
EVU: heat pump off
Additional heating device: hot water + heating
Current flow temperature: 38°C
Outdoor temperature cutoff limit: 21°C
Heating curve: 1.20
Min. target flow temperature: 15°C
Max. target flow temperature: 90°C

For hot water:
Target flow temperature: 0°C
Storage charging pump: off
Circulation pump: off
Legio. protection day: off

I am a complete layman, but isn’t a heating curve of 1.20 for underfloor heating way too high?? And does the adaptive heating curve have to be off?

Open all valves fully and set the heating curve to a relaxed 0.4. 1.2 is for radiators in a house from 1990.

38 degrees supply temperature is really cozy.