Is the attic always colder than the ground floor?

You won't get the perfect balancing where you bring the entire house to just the right temperature with all the radiators turned fully up everywhere in reality.

But that works with today's heating systems and houses...however, the balancing takes several weeks...so only if you do it yourself is the matter economical....

The ERR then does the final fine adjustment.

Well, ERR and "fine" are two different worlds...

The thermostat doesn't adjust anything either but only serves the purpose it is meant for, namely it is the emergency brake...meaning the heating circuits are throttled as soon as a temperature X is reached, and the rooms then fluctuate with the temperatures back and forth...

because in the attic you still have the roof as an "outside wall".

Exactly...in the attic you simply have more losses than in the ground floor...and these must be compensated by a higher flow temperature or higher flow rate.

Besides, I would adjust the valves of the individual heating circuits; you haven't mentioned those yet. Then raise the ones on the upper floor.

This is the only "correct" way...

By the way, ERR is very controversially discussed in other forums. I cannot say what is better; everyone has to decide that for themselves.

Also in all forums here...in modern and daily occupied houses ERRs are often about as necessary as a goiter...

Why should the flow rate be slower on the upper floor compared to the ground floor? That is physically untenable.

Depending on the length of the individual heating circuits, the resistance may be higher; additionally, the water has to be pumped higher, meaning without proper hydraulic balancing, the water definitely flows slower upstairs, which we also had before the balancing...

The heating engineer said that this is normal because there are many windows and it is always cooler in the attic. We should simply increase the flow temperature.

What is the contractual situation? Who sold what to whom and what was agreed upon? I mean, the scope of services should specify a design flow temperature. Then, under given external conditions, a temperature considered sufficient should be achievable everywhere with at most this flow temperature. It can therefore not be normal that it is "cooler" upstairs. It is normal that every room is as warm as the occupants desire.

Depending on the length of the individual heating circuits, the resistance may be higher, and the water also has to be pumped higher, meaning that without a proper hydraulic balancing, the water at the top definitely flows more slowly, that’s what we had before the balancing...

That is physically incorrect. In a closed circuit, the delivery height does not matter. The additional length of 2.80m is also not relevant in relation to the total length of the heating loops.

Moreover, I am very happy with my ERR; otherwise, how should I realize a staggered night setback per room? Then either nobody can sleep in the bedrooms, or you freeze in all the other rooms, or the window gets opened. I do not have a green conscience and I do not care about energy costs, but the practice has to work.

That is not physically correct. In a closed circuit, the delivery height does not matter. The additional length of 2.80m is also not relevant in relation to the total length of the heating loops.

Moreover, I am very happy with my ERR, how else should I realize a nighttime setback staggered over time per room? Otherwise, either no one can sleep in the bedrooms, or everyone else is freezing, or the window is opened. I have no green conscience and I do not care about energy costs, but practice has to work.

Okay, true, since the water on the other side also flows downward again, the delivery height should not matter. However, the question is whether the heating circuits are still different lengths due to the number/rooms or the size of the individual rooms. Our longest circuit is, for example, over 100m, the shortest almost 70, and there are indeed pressure differences...

Our heating technician also pointed out that the pump must be set to a certain minimum capacity to achieve the delivery height to the upper floor. When you adjust the capacity on our pump, a display in meters briefly appears.