Often a blank lightbox is due to attempting to load a page that cannot be loaded into an iframe
You can use your link in an online iframe editor.
If the container is empty, you can open up the Web Console to search for errors.
In some cases, you might find the following error.
Refused to display...[test page address] in a frame because it set 'X-Frame-Options' to 'sameorigin'.
The X-Frame-Options are configured on the server and control where and when content from the server can be loaded into an iframe.
To fix this issue, you can
- Change that setting on the server (if you have access), or
- Post your published content to that same server (if you have been testing locally to this point), in which case the content should appear.
Several big sites (Google, YouTube, etc) block the loading of content into an iframe, unless you use the specific embeddable URL.