A while back, we implemented a multi-domain (not subdomain) installation of WordPress by enabling the multi-user features and installing a multi-domain plugin. Once we got everything working, one of the issues we ran into was a login loop when someone was trying to login to WordPress on one of the domains. Even more strange, it was happening on Firefox and Internet Explorer, but not Chrome.
We tracked the issue down to the use of browser cookies for WordPress. We had to define the Cookie path within our wp-config.php file and then all worked well! Here's how to define your Cookie paths within your multi-domain configuration:
define('ADMIN_COOKIE_PATH', '/'); define('COOKIE_DOMAIN', ''); define('COOKIEPATH', ''); define('SITECOOKIEPATH', '');
Thanks to Joost De Valk for his input on this issue. It was a while ago, and I never stopped to thank him for his assistance.