It could be an issue with a overzealous blocker blocking not just YouTube video URLs but also a bunch of APIs and/or CDNs known to be used by YouTube, which als

It could be an issue with a overzealous blocker blocking not just YouTube video URLs but also a bunch of APIs and/or CDNs known to be used by YouTube, which also happen to be used by several other Google services. In Google's defense, they have no reason to set up a different API or CDN for each service they own and operate. That would be a maintenance nightmare.