First I've installed an OVMS (https://www.openvehicles.com ), which has a great diagnostic interface for the BMS. There, I could see the voltages of all 4 cells of BMU6 floating around, having values clearly out of any sensible range. If this happens too long the BMU will issue an error and hence charging will be stpped.
Having researched online I've found others also have similar problems. There are some good threads in forums:
There I've found some schematics and pictures of the BMS PCB, showing the IC LTC6802, looking up the datasheet reveals it's a battery cell monitor. Thats's probably the defective chip.
In some forums I've found great schematics of the iMIEV's battery. They later helped me a lot.