Surprisingly enough the CHAdeMO DC fastcharge controller and even the vehicle control unit (which was at the front end connected to the Ampseal connector) seem to be still working.
For safety, first step before proceeding was to do some measurements while wearing protective gloves. Potential between the middle of the rear pack and chassis of the car was 140V. So there apparently was a short to ground in the front box (as expected). Cutting the cable going into the front box removed that potential.
Next step was to take the battery management system master out and connect it to my computer. As expected that worked. I immediately checked the BMS error log. Nothing in there that could shine any light on the root cause of the fire. Only some old errors from configuration mistakes during testing before we travelled to Sweden.