HippoCampus playlists were designed with the option to include content from other sites across the internet. However, some sites are concerned about "clickjacking" which is a malicious effort to redirect users to unexpected content. These sites may block other sites -- like HippoCampus -- from displaying their content in frames of any type using the X-Frame-Options HTTP response header.
When a site decides to set this block in place, all sites but their own are unable to load the content in a frame, even if the intent is benevolent such as with HippoCampus. Unfortunately, when this happens, the only on-screen notification is from the browser itself, and we do not control the error message. You can usually tell that this is the case when you have checked for a security conflict, but the content from the external site still refuses to load.
Unfortunately, there is no fix that NROC can implement for this problem. It is up to each website to determine their own security policies. As an end user, you may wish to bring this issue to the attention of the website's technical team and let them know they are blocking legitimate use of iframes.