Verify the rotary controller is working correctly in the Service Menu. Watch the amount of steps per click. You may simply have a defect rotary controller.

Otherwise, monitor the ibus with NavCoder to verify the ibus messages show the amount of turns/clicks occuring. If so, BMBT is OK and problem lies in Nav.

If not, BMBT controller is defekt.

If BMBT is OK and problem is in nav, you can reload the Nav OS yourself, thus performing a reflash. Simply insert a BMW OS disv (google "BMW V32") and follow the onscreen instructions.