Hi Giustiniano,
Looking at your photos it is clear that your autopilot is a much more recent installation than mine (1999) and I can offer little assistance.
However, from your description it seems like mechanical wear or failure in the stepper motor or the annuular. Since your autopilot is younger it may still be possible to obtain replacement parts, or even have it serviced.
May I suggest you find out if Raymarine have a service centre in your country, and send them email describing the fault and ask for advice.
Jeremy Rowley