Thanks for the details.
Some major sites (e.g., Google, Facebook, Twitter, Youtube, etc) block the loading of some of their pages into a lightbox.
Depending on the content, there may be an "embeddable" version that can go into a lightbox.
Alternatively, You can bypass the lightbox for those links: How to Bypass the Lightbox for Individual Hyperlinks
YouTube can be used with the lightbox as a hyperlink if you simply edit the address.
See this answer: