ReAuth=1 is required when your login Cookies are no longer valid, WordPress will force validation for your browser.
if ( $force_reauth )
$login_url = add_query_arg('reauth', '1', $login_url);
- Add reauth=1 flag to login url when auth_redirect() redirects to wp-login.php after the auth cookie fails validation wp-login.php
clears cookies and forces log in if reauth=1.- If reauth=1 wp-login.php does not attempt to redirect to wp-admin, even if the cookie seems good.
Basically, this forces reauth/login whenever auth_redirect() does not
think the user is logged in. This should resolve the situation where
one cookie seems good but another does not.
— Ryan via Trac 12142
To fix issues with this you can clear your cookies and change your Security Keys, or use wp_set_auth_cookie.
Some reference:
http://digwp.com/2010/09/wordpress-security-keys/
http://codex.wordpress.org/Function_Reference/wp_set_auth_cookie