I'm having the same issue as sixfootjames and it looks like I just got it to work.
Just to preface, the YouTube videos were displaying and playing fine on my client's site with my site as the source last Tuesday. Today, when I logged into my client's site to check up on things, the embedded YT videos (iframe) weren't displaying at all but still worked on my source site.
Not good. So here's what I did:
I deleted the secret URL from my client's site > Save Changes > pasted the secret URL back in > Save Changes.
Now the videos are displaying again in the client's site. Hope that helps.
* By the way, between last Tuesday and now, I had installed the WP Select Box Navi plugin on my source site which added a select box at the bottom of the WP Help hierarchy menu. This may have been what triggered the issue for me. I deactivated the WP Select Box Navi plugin and then went through the steps I outlined above.