Iframe widget is broken for mx 5
I have an update that works!
I get the same result for the iFrame widget when there is no URL in the attribute of the enclosing entity for the widget. Try creating a page without the iFrame widget where you can ensure that the URL attribute is populated. Then create a separate page that opens the URL in the iFrame widget. Hope this resolves your problem.
If so, you could create a microflow that checks the URL attribute before opening the page. This would prevent users from seeing the progress message you did.