Login loop on when logging into mapped domain

WPMU base: oblogs.co.za
mapped domain: overcoming-depression.org
admin mapping: original domain
login mapping: original domain
cross-domain login: true

I go to oblogs.co.za/wp-admin and login fine.

Go to overcoming-depression.org/wp-admin.
Get prompted for login credentials.
Login redirects back to the login page and prompts for credentials again (password and username all correct).
However, I AM logged in.

In the URL there is an errant reauth=1 that bumps me back to the login page after login.

Any ideas on how to fix this. I've had one client who just keeps logging, then changed their password and swears they can't get into their site. I know they can, but for a user, that one step is too much (and unnecessary).