Since WordPress is configured to respond at www.yourdomain.com and you've set it up as multisite, WordPress is looking for a different installation on the www subdomain (not knowing that they're configured to be one and the same). When it doesn't find a separate WordPress install there, it automatically redirects to the sign-up page of the 'primary' site.
This fix for this is to tell WordPress that you don't want it to redirect to the sign-up page when it can't find a separate WordPress install. Login via FTP or use the Plesk File Manager to edit the wp-config.php file in the folder to which WordPress is installed (for most this is simply in httpdocs). Anywhere you see define() statements, throw this in right after them:
Be sure to replace yourdomain.com with your actual domain name. You may need to clear your browser's cookies to tell it to forget about the incorrect 301 redirect. Visit www.yourdomain.com to confirm that the redirect now functions as you expect it to.