The German BFU has released its intermediate report (sorry, in German only):
https://www.bfu-web.de/DE/Publikatio...ublicationFile
The intermediate report only lists the known facts.
According to the BFU, another pilot flying an airliner on approach to nearby Stuttgart airport reported severe icing conditions at about the same time as the C510 was approaching Friedrichshafen. The airliner had very quickly accumulated 2-3cm (1-1.5") of ice on its windscreen.
A B1900, which landed in Friedrichshafen about 45 minutes after the accident, was also covered with thick layer of clear ice. According to ground crews, the icing was so bad, they required about 4 times the normal amount of deicing fluid, to prepare the B1900 for its next leg.
The BFU has found no indications of any malfunction. Neither do they mention any flap issue. The C510 entered a steep dive at the very moment, as they were about to capture the ILS - while they were still flying at a speed of about 240kts.
Eventually they crashed at a very shallow angle - after clipping trees about 1km from the final accident site. So, it seems the C510 had already recovered from the steep descent.
So, no clear explanation yet. We'll have to wait for the final report...