When a landing page with an embedded video is published, you receive the following error (or something similar, based on what service the video is coming from): Page contains hosted content (iframe) that is invalid. The missing file is www.youtube.com/embed/YuX1hsGRf2Y.
Check your embed code that is taken from the service. It is likely missing the 'http:' or 'https://' from the src= element.
To fix, see whether the page the video is contained on has https: or http: and place it before the two forward slashes. Save the page and this error does not appear when publishing.