Thank you for your help!!!
The problem occurs not for each movie, but for example this media to url link: http://www.youtube.com/watch?v=MKyJzXDWaU4
Dolphin doesn't show correctly but corrupted. For me would be enough get the window to select the preffered app to play (f.e.YouTube) as I said some post on.
I think that I have the Flash Player installed on (i have an icon called Flash Player settings into Apps menu) because I can open others media files...
I don't think that it's a installation problem otherwise I couldn't open each file...