When you have launched a piece of content in the Yahoo! Video Viewer window, and then logged out of Yahoo! in a separate browser window, the Yahoo! Video Viewer window does not automatically close.

So when you try to launch another piece of content from the Yahoo! Video Viewer window, it needs you to log back in to Yahoo! so it can verify your Yahoo! subscription. Simply clicking the "Sign in Again" link in the viewer window will let you sign in to Yahoo! so you can continue to enjoy content on Yahoo! Video.