Something just occurred to me, Francesco: In another thread, Dave Phillips mentioned that OSC with the loopback IP address (127.0.0.1) was not working between Ardour and Iannix in his Linux environment (I forgot what distro he was using — it might have been Ubuntu). He reported that Paul Davis (the author of Ardour) acknowledged this problem and thought it might be trouble with liblo. Paul recommended instead using the actual local IP address (usually something like 192.168.001.030). This fixed Dave’s problem.
The troubling thing is that you see the correct incoming OSC messages in the Iannix inspector. Did you double-check that? Anyway, you might want to try the IP-thing.