Hmmmm. A wet ground wouldn't cause these problems. However, a bad ground certainly would.
Your car's problems seem to be very general in nature. You stated that the car cranked very strongly. That would seem to rule out a battery problem. During the rain storm did you drive your car through any puddles? Could the alternator have ingested some water?
Try this: disconnect the ground lead from the battery disconnect they positively from the battery short them together for 10 to 15 minutes reverse the steps. By the way this is a BMW approved process.