Server notifications v2 UNSUCCESSFUL_HTTP_RESPONSE_CODE

Hello. I'm currently implementing Apple Notification v2 to prepare for refunds for in-app purchases, but I'm not receiving requests from Apple servers to my backend server.

I've applied HTTPS (TLS 1.2) and correctly registered production/sandbox notification URLs on App Store Connect.

After requesting a test notification, when I check the status of testNotificationToken, I receive an UNSUCCESSFUL_HTTP_RESPONSE_CODE as follows: {"signedPayload":"......":[{"atteptDate":1752128001970,"sendAttemptResult":"UNSUCCESSFUL_HTTP_RESPONSE_CODE"}]}

The endpoint for receiving notifications is set to accept POST requests with application/json format, and it responds with 200 (OK) without any content. However, Apple notifications are not coming through.

Could anyone help me with this issue?

Server notifications v2 UNSUCCESSFUL_HTTP_RESPONSE_CODE
 
 
Q