A user has an account on one (and only one) SE 2.0 site, and would like to create an account on MSO. They're using the Stack Exchange OpenID provider.
I've instructed them to visit http://meta.stackoverflow.com/users/login and click the "login with Stack Exchange" button, which normally brings up this page, where they can enter their credentials:
Everything below the horizontal rule under "Log In" and above "Don't have an account" is in an encrypted iframe. For this user, the iframe does not show up.
What could cause this? How can we work around it?