Recently we were getting the below issue for our Marketing Forms hosted on an external website on a particular authenticated domain. The pages were working fine in other authenticated domains.
“The domain where this form is embedded might not be enlisted as a domain record for externally hosted forms. Otherwise, a browser extension or plugin may be blocking this form from loading. Review and reload form to continue.”
This particular domain was also shown as authenticated in our Dynamics 365 marketing.
Our website was e.g. https://www.domain.kiwi/ so we had domain.kiwi authenticated in Dynamics 365 Marketing.
Eventually, we had to raise a Microsoft Support Ticket and were instructed to add/ authenticate www.domain.kiwi (with www.) also, doing so resolved the issue for us.
Also, there seemed like a UI bug in the Marketing which will cut out “www.” automatically when trying to authenticate the domains.
Hope it helps..
Nishant Rana's Weblog 
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