Had a similar problem many years ago on my old Rover 400.
Apparently the sensor picked up the serrations in a ring that rotated with the wheel – my ring had split and it slipped, which was picked up by the sensor. I believe that it was a simple function like that which detected if the wheel was skidding and operated the ABS/ASR
May be completely irrelevant to your problem as systems may be quite different and I have not been out to check what is on the Ducato
Fred