To bring this story to an end:
I disassembled the brake as far as possible.
It is a simple electric powered magnet completely embedded in a resin, thus you can not check the wires etc..
It holds (usually) a ring of metall the moment you use the furler.
The ring is (all the other time) pressing against a brake disc (plastic, connected with the shaft of the electric motor via a gearwheel) by 8 small spiral springs - arranged in 2 circles.
I checked the wires going in to the magnet. There are appr. 25V when furling. But the magnet was not reacting. I „helped“ him by pressing and then it worked.
Thus I assume the magnet is getting „old“, reduced the springs from 8 to 4 (keeping the outer ones).
Now it is working fine again and there is still enough pressure to hold the foil.
Be aware of the 3 screws on the outside of the device. They are for adjusting the space/ pressure to the brake disc. So when there is slip (e.g. the main is furling out itself in a gust), you have to turn them a bit out. When it seems like the brake is not releasing the furler (enough), the screws need to be tightened a bit (pressing the disc more to the springs and the magnet).
I assume that was what Alexander (A54#156) meant with, „It (the brake) needs a carefull synchronisation with the furler engine.“
You can take the brake out of the furler by only disconnecting the down cap. The brake is the first thing you see when the cap is open.