Answer
The workflow engine's REST calls originate from the middle tier's own network path, which can be subject to different firewall rules than whatever machine and network path a testing tool like Postman is running from — a working Postman call doesn't rule out a firewall blocking the middle tier itself from reaching that same URL; check firewall rules along the middle tier's actual outbound network path specifically, rather than assuming the URL or endpoint configuration must be at fault just because it works from a developer's own machine.