Login problems on inline frame

I have a project on a site. The page is shown in a htm page as an inline frame (<iframe/> - a feature of Frontpage) Using this feature is nice as I do not need to create a header and footer and the database part integrates well with the rest of the site.
Problem however:
If a user logs in to the project (created with PHPRunner of course) the login is not accepted - no errors, just returns to login page. This seems to happen on older versions of Internet Exporer only. When I open the link in a new page directly (not as inline frame), the login works fine.
Do anyone have an idea why this would happen. It must have something to do with some code clashing.
View the page at: Link
Thanks for help!