If you are testing the notification endpoint in the (Customer Area > Account > Server Communication > Troubleshoot) and the plugin returns with a 301, 302 or 401 error instead of an [accepted] response, there might be something wrong in the notification endpoint settings.
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.
For more information and troubleshooting refer to Why am I not receiving notifications?