Upgraded from 1.6.x to 1.7, cleared cache and cookies, and now going to the admin login form results in a 501 error. The only output is the header that you would normally see once you’re logged in to Magento, followed by an incomplete paragraph tag with the “super” class add on to it. No login form.
It honestly looks like Magento is attempting to load the actual Admin area and not the login page. Clearing cookies and the cache does not help, and we’ve never touched any of the core Magento files or admin templates.
Incidentally, I can login just fine if I go to the MagentoConnect Manager, enter my credentials, and then click on the “Return to Admin” link.
EDIT: Figured it out. It was an issue related to line 33 in “/adminhtml/default/default/template/page/header.phtml”.
Issue and fix found here: http://www.magentocommerce.com/boards/viewthread/289621/