I've been working on setting up alternate URL's to direct to advertisers. But I ran into a problem when someone went to mydomain.com/advertiser instead of http://www.mydomain.com/advertsier (I still don't have this working) all in all going to either address should ship you off to my partner link with my advertiser - http://www.advertiser.com/mydomainname.
Anyway, what it was doing was giving me this error - You are logged in already. No need to register again!
The site you were looking for, http://mydomaincom.www.mydomain.com/, does not exist.
To fix this problem if you've set things up wrong and set the site up with a www as most people want.
Go into your database.
Open WP_Blog table - you'll see http://www.yourdomain.com there - edit it. Remove the www
Open the wp_site table - you'll see yet again http://www.yourdomain.com there - edit it.
Remove the www
Close your database
Open wp-config.php - Remove the under DOMAIN_CURRENT_SITE - Remove the www as well there.
This will fix the problem. To keep your www address set that up with your redirects in your .htaccess file.