Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Create a Custom Web Clip Widget
If you want some live video from YouTube or another site to constantly run on your Mac, but don’t want to keep your browser always open, then a Web Clip widget is exactly what you need. To create it, do the following:
1. Go to YouTube.com or another site that hosts the video.
2. Navigate to the page with the video you’d like to create a widget for.
3. On the Safari toolbar, click the Web Clip button.
4. The webpage will be dimmed with only one highlighted box. Drag this box over the video, drop it, and adjust its size to that of the video preview using your mouse.
5. Click Add when ready.
Once the widget is created, Mac OS will open the Dashboard so that you can see the results. To play the video, click the play button in the widget.
There are also special Terminal scripts that enable you to drag the widget out of the Dashboard, place in onto your desktop, and watch the video while you’re working.