When testing the notification endpoint (Account >> Server Communication >> Troubleshoot), the plugin returns with a 301, 302 or 401 error instead of an [accepted] response.
To solve this, please check if the username and password for the notification endpoint are set properly. If you are configuring the endpoint with http, try using https instead. If you set SSL version to SSL, try setting it on TLSv1.2 instead.
Also check our FAQ page on "Why am I not receiving notifications?"?