I've attached all the cables of the battery to the car, except the orange high voltage cables. This allowed to test the BMU and observe the cell voltages using OVMS again.
Be aware, this triggered an airbag warning, wich needs to be reset by the garage. The warning is due to powering the car (attach the 12V battery) with disconnected airbags. The airbags are disconnected because I've taken out the drivers seat to gain access to the HV plug of the battery.