A final report.
The guy who sold CMU boards on ebay had the right board for me.
I have no garage so it was a bit problematic to change the board, but this weekend I could do it at work.
It was easier than I thought to remove the battery and change the board.
Now the car is running fine again. But in the process of dealing with this problem I managed to reset the BMU.
I guess that I now have to cycle the battery a few times before I get a correct reading on the battery status.
I tried to run the battery low today but at about 20% I suddenly got turtle mode. After just a kilometer the power was reduced to less than 2 kW.
I managed to crawl in to the parking lot at the supermarket and charge the car.
After 10 minutes of charging I got two dots on the SOC meter and could drive home without any turtle mode. I guess that the BMU is a bit confused at the moment.