Insufficient effect of air conditioning

The effect of air conditioning in my A8 (D2, Bj. 1999) is deficient. The air is cool, but the volume flow is less than 50% of the normal volume even when the blower is fully switched on.

Furthermore, it is noticeable that when the air conditioning system ("eco") is switched off, the volume flow is ok. Immediately after starting it is also ok for a very short time, after which hardly anything gets through again.

The filters were exchanged on the first visit to the friendly one. No effect.

On the second visit to the friendly one I was informed that this old vehicle already leaks in the rear area. In addition, part of the air is blown out in the rear passenger compartment. But I don't think so, because first of all, after switching off the Klima system, a lot of air passes through and second, the ventilation in the rear was switched off.

In the ÖAMTC, the error memory was read out: "Circular valve actuator blocked or without voltage".

Can this be the cause of the lack of air? Does anyone have another tip?

Repair instructions are useless - have two left hands. But what does such a repair cost?

I'm grateful for every hint.

Best regards from LInz, Ernest