Thank you for your answer but it didn't help me to solve my problem. I searched for others experiencing quite the same problem before opening my thread but I can't solve mine reading other's threads.
I now add to my first message that PHD2 freezes not only after the SGP autofocus routine but if it manages out to carry on and go over the AF routine (I don't understand how and why sometimes it does it, some others it doesn't), it also freezes after the first image taken. I remark that PHD2 does not stuck, it is still "alive" during this happening. It stops drawing graphs, but if I push on the stop icon choose another star for guiding and start guiding again it goes on guiding until the next picture has taken.
Now I uninstalled PHD2 and re-install it back. I have to wait for another good sky to test if something changes.... but I think it would not only after this simple operation.... Thank you for your time and help.