If you’re behind a WatchGuard firewall, and your /wp-admin/ page shows up white / blank, then your firewall may be the culprit.

The default Header-Response for an http proxy is set to strip. Which prevents the admin page from redirecting you to /wp-login.php

You can test this by manually typing in http://site/wp-login.php. Once you log in (it wont redirect you), type http://site/wp-admin/ and your dashboard should load.