There is one interesting thing about this, I can watch youtube videos posted at other sites (Facebook) and the advertisements play fine. It's only the actual video that's having a problem. For me anyway.

I have the same problem, but I can watch the videos from the channels and in the "pop out" option I choose to watch them on "pop out" (the option at the right of the quality selector)

Problem is related to Flash and Video drivers. Try updating your video drivers first of all.

As a temporary workaround try hovering your cursor over the place where the pop-out button should be below the YouTube video screen, then left click there. The video will then display in the pop-out screen.

Another temporary work around is to go to settings. Right click on the video that should be playing and choose settings. A window will pop up. The first tab (the tabs are the icons at the bottom of the settings window) should say "Display" on that tab un-check "Enable hardware acceleration."

If you cannot get to setting while in Youtube (like I couldn't), go to a site with a Youtube video in it like Facebook. Follow the same steps as above and it should work. It did for me anyway. Let me know if this helps.

I had this exact problem. Uninstalling/reinstalling flash had no lasting effect.

Uninstalling and reinstalling the most reccent video drivers fixed this for me. I'm using an ATI Radeon HD 5800 (http://support.amd.com/us/gpudownload/Pages/index.aspx)

This has solved my issue. Thank You!