You can also try 'resetting' the sensor by turning the steering wheel 'lock to lock' (full left, then full right, then left) two or three times. It sometimes forgets where it is.
Whenever someone has unexplained sensor errors I first think of the battery. A slightly low battery ( < 12v) can often cause electrical 'gremlins' in a W210. Anything less than a top of the line battery less than 4 years old is suspicious. FYI - the battery needs to be load tested while out of the car's circuit.
Hope that helps some.