This error usually means the API URL is incorrect or returning an HTML error page instead of XML/SOAP. In Magento 1.9.3.3, make sure you're using the correct SOAP API v1/v2 endpoint (e.g., http://yourdomain.com/index.php/api/v2_soap/?wsdl), and that API access is enabled in the admin panel. Also, check for server issues like redirects, firewalls, or missing PHP SOAP extensions that might cause incomplete responses.