I deinstalled the powerkernel and all fcam compnents (drivers and linked apps), then rebooted and reninstalled everything again 1) powerkernel 2) special driver 3) fcam and 4) linked apps (HDR and lowlight).
Now it's all up and running again.

Other question:
Does someone have an application (both for Linux on the PC and on the N900) to view and process the DNG raw files made by Fcam? Currently I can only use Evince ducument viewer to see the picts, but's that not very handy and not a real processing of a camera raw.