Cannot Login - Redirects Back of Login Page

I have a dilemma with domain mapping plugin...

In an incognito browser (cleared cookies), I try to access my wp-admin via mapped domain. The login fails and redirects back to the login page. As a result, I cannot get into the backend. I also tried logging in first through my primary domain, then trying to access the backend of my mapped subdomain, but have no luck... it gets stuck at the login screen. The only way around this is I go in phpMyAdmin and find the domain map table and wipe the mapping.

So to illustrate, this is what happens:

1) Let's say I try to go here: MyMappedDomain.com/wp-admin/

2) I cannot login and get redirected here:

http://user.primarydomain.com/wp-login.php?redirect_to=http%3A%2F%2FMyMappedDomain.com%2Fwp-admin%2F&reauth=1

Any clue as to how to fix/troubleshoot this issue?