The attic heats up very strongly

We have an uninhabited attic with insulation between the rafters made of insulating wool. Between the attic and the upper floor, there is another insulated ceiling. Nevertheless, the attic heats up extremely in summer. Is this normal or how can I remedy it?

That is unfortunately normal with intense sunlight exposure (so almost always in summer). The insulation can only delay the heating up, but once the heat is inside, it naturally cools down more slowly when it gets cooler (usually at night). That means either ventilate at night (be careful, though, as this can also lead to moisture deposition). Or just ignore it. I assume it is unoccupied? Do you store sensitive items there?

Double insulation has its pitfalls - the problem with heat you mentioned is one of them. Therefore, I am replacing "Dennoch" with "Deswegen".

Our problem is the central ventilation system. It is located in the attic and already brings in 30 degrees supply air at an outside temperature of 20 degrees. Probably because of the heat in the attic.

Does anyone have a solution for this?

Enclose and insulate with vacuum insulation panels (will be expensive). Or just very thick with other insulation material. Install air conditioning - either only in the enclosure or for the entire attic. And then have it continuously cool at a low level (not only at full power during extreme heat).

Is that normal or how do I fix it?

Yes. The heat eventually penetrates even the thickest insulation. Air exchange can reduce the problem, but insulation works not only from outside to inside but also from inside to outside. Once the roof is "heated through," the heat is well retained. An air conditioner is one option - perhaps a bit too much for an uninhabited attic. I would simply leave it be – and then just pay attention to what is stored there.