YouTube videos and other streaming media will fail to load if the action for the Streaming and Download category in URL Filtering is set to Prompt.
With the prompt action, when users first visit YouTube, they'll be presented with a warning page. They can click the "Proceed" button to continue to YouTube.
However, when users try to view any video, they'll only see the loading icon indefinitely.
1. Change the action for the Streaming and Download category to Allow or Monitor if displaying the warning page is not a priority.
2. If you only want to allow specific streaming sites, but use the Prompt action for all other sites in the Streaming and Download category, you can create a Custom Category policy with the Allow or Monitor action.
The example below is a Custom Category for YouTube with the action set to Monitor. At the time of this writing, googlevideo.com is the TLD used by YouTube for streaming video.