The issue is likely you are doing SSL on not on port 443.
Hence the Postman warning. Either that or it’s not using the cert you think it is
I use Let’s Encrypt certs without issue with webhooks. But I don’t use non standard ports.
The issue is likely the use of port 6001. (use NGINX to proxy pass if you need to and it’s nicer to move SSL termination to nginx/apache/whatever than adding it to your express server in my opinion)