Indeed the problem was created by the TURBO function. Not only on YT, but for any kind of video, Opera refused to open automatically the video and asked to click in order to activate Adobe Flash Player.
So just uncheck this function and everything will work normally.
Anyway this problem created me a lot of stress. I had to uninstall and then to reinstall both : Opera and Adobe Flash Player.
Finally I found the solution on this forum.

Thanks a lot guys !!!