Browser redirects back to the password page upon submit

We have the plugin installed on a network site. The intention is that users have to enter a password in order to view the site. We have noticed a problem in Internet Explorer (sometimes Opera as well) where the login page redirects back to itself. This loop doesn't ever seem to break, though if you remove the query string that gets appended to the URI then you are sometimes able to view the site. Can you give any guidance on this?

URI: http://