I got in touch with the German dealer. They suggested to replace the unit with a new model. They also made an offer to replace the electronic boards. But the price was high. So for me both options are not the way to go in these times.
From what I can say, the regenerator part does not look like to be broken. The green LED on one of the driver boards is continuously on in times when the Power Plant is working properly. There are times when the device is running several days. Then (for no obvious reasons) the relays begin to click in short succession. For me the problem looks more like problem in the control circuit. Also the device is able to drive an electrical heater with 2000 Watts without any problems.
In most cases (when the device is not working properly) the relays and the control logic simply does not connect the regenerator output to the device output and the relays are switching between regenerator and the the plain voltage. When switched on, several tries are needed to get into a stable function. This also makes me think that the control board might be the problem, and not the regenerators power stage.
It would be very helpful to track down the problem if you are able to provide an electrical drawing or at least a service manual with voltage values and waveforms to be expected at given measuring points. I have a master of electronics, so I think that will have a chance to trace down the problem with this kind of information. Any other ideas are appreciated…
Thanks in advance,