Login redirect loop only from subdomain blogs

Most of my users log in from the login widget on the root blog's edu-clean theme, which works without problem. However, a user tried to log in from their mapped domain's Meta login, and they get a redirect loop


I have buddypress and bbpress integrated without issue, so there are the usual auth salts etc. in wp-config. I added the admin_cookie_path just in case (suggested here: http://mu.wordpress.org/forums/topic/9872), but it makes no difference that I can tell. Of course cookies deleted before each test.

define( 'COOKIE_DOMAIN', '.bestagentblogs.com' );
define( 'SITECOOKIEPATH', '/' );
define( 'COOKIEPATH', '/' );
define('ADMIN_COOKIE_PATH', '/');

Could my cookie_domain be part of the problem? Any suggestions appreciated.