I've posted a similar observation... no response, no replies... The error persists, and getting work completed is painfully slow.

My "server" endpoint is set up as an AWS lambda (using TLS 1.2). I can access this endpoint without issue. I am using Node.js in the client-side test code to request the test notification. The response from requestTestNotification() is an object with a testNotificationToken. I use this token to call getTestNotificationStatus(). The response from getTestNotificationStatus() contains 1 send attempt and the sendAttemptResult is: 'UNSUCCESSFUL_HTTP_RESPONSE_CODE'. This has never worked for me.