Thanks for the reply. I've talked with the dealer via email. They suggest stupid solutions like removing the entire gasket permanently, which will decrease the performance. The dealer installed the centerboard when the board was brand new so it is not an installation issue. I could not fix the problem with a soap. Also I already have opened the cassette and made pictures. There are some scratches suggesting friction between the cassette walls and the centerboard, but the main problem is the friction between the gasket and the centerboard. The problem is widespread, because there is a forum thread with many people reporting same issue: search "Phantom 320 Daggerboard Stiff ".
I sent today an email to the dealer with a request to help me contact the warranty service. I live in California and I think trident sport are the distributor for USA. I just checked their web site and they have also a warranty claim form for starboard products. I have a bad feeling about starboard support. They do not provide any phone numbers or emails on their web site. The web form tthey provide is well hidden into the menus and they did not respond to the submitted request. The "special" "ask your team" forum does not accept new registration (administrator disabled it). The forum welcomes the latest new user Antoine Lefebvre, who has not been active since October 2012. That means that the forum has been closed at least for the last 4 months. I expected better for a product that cost me more than $1500.