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< 			$destination = 'http://' . $current_site->domain . $current_site->path . 'wp-signup.php?new=' . str_replace( '.' . $current_site->domain, '', $domain );
---
> 			$site_domain = preg_replace( '|^www\.|', '', $current_site->domain );
> 			$destination = 'http://' . $current_site->domain . $current_site->path . 'wp-signup.php?new=' . str_replace( '.' . $site_domain, '', $domain );