I checked your configuration with https://webhook.site and it works fine. There is something peculiar with the API you upload to that rejects the POST request from n8n.
I have only one suggestion left to try. Replace the Athorization header with Header authentication creds. That is, create the generic creds Header Auth and use that for authentication removing the corresponding header in the node configuration.
Thank you very much for your time and suggestions.
Unfortunately it did not work. I could not make this happen. I believe that API is not working as expected, so I abandoned this.