Sharepoint doesn't redirect to default.aspx

While logged in as a User in Sharepoint 2010, if I navigate to the main page of our Sharepoint (http://sharepoint) it asks for my username and password but never actually logs on (stuck in a loop). If I navigate straight to http://sharepoint/default.aspx, it works fine.

If I'm logged in as the System Account admin, then navigating to http://sharepoint redirects me automatically to /default.aspx.