Some websites such as Facebook, Instagram, Pinterest, YouTube, and more do not allow their content to be loaded inside an IFRAME, which is what is used inside Redframe’s modal window.
Redframe maintains a list of websites that are known to be incompatible with IFRAMEs and will automatically detect if an external page is going to one of these sites. If an incompatible link is detected, the page will load in a new tab rather than inside the modal window.
Odds are that your external page link is to a site that is incompatible inside IFRAMEs, but that this site is not currently on Redframe’s list of incompatible links. If you think this is the case, please contact Redframe Support and ask them to add the URL to the incompatible links list.
For more information about external pages, read Create an External Page.