iframes are an html element inserted normally, I've often used them in my surveys. So, if you are getting a not found errors, there could be issues with the code or maybe the source website is blocking the iframe.

I don't have issues embedding them into surveys but for some reason the error only occurs when embedding into an End of Survey Message

As an update, for some reason the only website I can get to work in an iframe (anywhere within the survey) now are Qualtrics websites such as qualtrics.com

Are you on safari/mac? I recently had a few respondents complain that they could not see the embedded videos (youtube via iframe). Even though they were working fine on browser/systems.

Hi ahmedA. I am on Windows 10 using Chrome. This issue was recently brought to my attention by a developer at our University who encountered this problem. I wonder if it could somehow be specific to our license.