Redirect Loop on logging in on Child Site

We can't login to any of our child sites we create using New Blog Templates. After we click the "login" button we're just redirected to the login form.

Please note we are not using a subdomain, but are using a regular http://www.ownsitename.com domain. Otherwise everything looks great (minus some missing product images).

For domain mapping we are using: "WordPress MU Domain Mapping"

Any idea why the login would be breaking?

Thanks