Ciao Piero and All,
I had about the same problem which came from the fact that the square end of the cable connected to the rearwheel was not long enough to be driven by the speedo "gearbox".
Everything looked OK when not connected to the rearwheel but didn't work once fitted.
I fixed it by shortening a bit the cable housing. From my memory, there should be about 10 to 12 millimeters of cable protruding out of the housing at the rear wheel end.
Hope it helps.
Laurent