I have subdomains configured on my site http://www.domain.com. I can access http://www.domain.com/wp-admin fine. However if I go to sub.domain.com/wp-admin
it sends me to this page
http://sub.domain.com/wp-login.php?redirect_to=http%3A%2F%2Fsub.domain.com%2Fwp-admin%2F&reauth=1
It never allows me to login to the wp-admin on a subdomain. However I can visit the page with no problem. The users cannot access their dashboard as well.