WordPress embedding not working ?

Hello

Have a problem : when WP 4 came, I tested successfully the embed feature.

Now, even with NO plugin at all, when I paste a YouTube link like https://www.youtube.com/watch?v=H-IU6IYM800, it begins to “embed” (I see a little animation progress bar) but… it only displays the direct link in the visual editor.

Do I miss something like a checkbox or something ?
Why can’t I embed YT video anymore ?

Which requirements are needed in webhost config to emebed video for instance ?

Thanks ??