Thank you PDB for your comment on GPS : this was may be the real cause of my problem (on top of the FTDI / Prolific conflict)
I explain : I have a Celestron CGE Pro mount. Because that mount is not included in the Celestron list displayed by KStars, I chose "Celestron GPS" .
Somehow it worked but fairly frequently KStars crashed at the end of the session .
Because of your comments I replaced "Celestron GPS" by "Celestron NextStar" and this doesn't happen anymore.
Thanks again