I ran into the same issue. When opening Postman console it gave the error message “Error: unable to verify the first certificate”.
This clued me in that the server side certificate was not complete. I combined my root cert with my intermediate to provide the entire cert chain and then provided that as my ssl_certificate.
cat root.pem intermediate.pem > chain.pem
Might be to late for you, but hopefully this helps someone else running into this.