Is the attic always colder than the ground floor?

If we open all the thermostats completely, we will also have a temperature difference between the ground floor and the attic. This is also logical to me, since in the attic you still have the roof as an "outer wall." In addition, the water in the supply line may also cool down a bit due to the longer distance from the basement to the attic, despite insulation.

If we open all the thermostats completely, we also have a temperature difference between the ground floor and the attic. It is logically certain to me since the attic still has the roof as an "outer wall". In addition, the water in the flow line may also cool down a bit due to the longer path from the basement to the attic despite insulation.

That's exactly what I already said. It's simply logical, and on top of that a 1 degree difference...

Increase the flow until it fits upstairs, and then adjust accordingly downstairs. Problem solved.

In most cases, the bedroom should also be a bit cooler, while the towel radiator in the bathroom still heats well... What surprises me a bit are the flow temperatures of 32 and 35 degrees? We have 28° flow @ 0° outside and meanwhile in the living room between 23 and 24 degrees...

If I were you, I would check one more thing before increasing the flow temperature and throttling the rooms in the basement and ground floor.

Water follows the path of least resistance, which in your case is the basement and the ground floor. Therefore, I would check whether the pump delivers the required amount of water to fully supply the heating circuits on the upper floor. If it does not, you have full flow in the basement and ground floor, but the flow slows down on the upper floor.

If the hydraulic balancing has been done, the calculated flow rates for all heating circuits should be available. Add up the flow rates of all heating circuits and then check whether the pump can deliver the required amount of water with the current settings. If not, increase the pump by one stage and observe the temperature on the upper floor.

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

Water takes the path of least resistance, which in your case is the basement and the ground floor. Therefore, I would check whether the pump delivers the required amount of water to fully supply the heating circuits on the upper floor. If it does not, you have full flow in the basement and ground floor, but the flow slows down on the upper floor.

Why should the flow slow down on the upper floor compared to the ground floor? That is physically untenable.

In addition, the water in the supply line might cool down a bit due to the longer distance from the basement to the attic despite insulation.

To measure this cooling, you would need a highly precise measuring device. The heat loss on this section is practically zero.

In most cases, the bedroom is supposed to be a bit cooler, and the towel radiator in the bathroom still heats well... What surprises me a bit are the flow temperatures of 32 or 35 degrees? We have 28° flow @ 0° outside and meanwhile in the living room between 23 and 24 degrees...

Do you perhaps have a closer pipe spacing because of the heat pump? At least that's the case for us. Then no such high flow temperature is needed.