i have this problem, and it seems that i'm not alone.
If i embed a simple apps script form with some textarea,buttons and labels in a custom web url www.mydomain.com/mypage...the page simply don't show nothing and the Chrome console report that error:
Refused to display document because display forbidden by X-Frame-Options.
Instead if i see the page from the original url:
https://sites.google.com/a/mydomain.com/mysite/mypage
i see the apps script correctly.
Anyone solved that problem or can tell me a workaround?? Thank you