Actix-web debugging

I've been struggling all day to get my actix-web server working with my actix-web client. A curl POST request from the console works, but the same request from the client returns a 404.

I'm not asking for help on this particular problem. I'm sure it's as simple as a typo that's right in front of my face that I just can't see. What I'm looking for is general guidance in debugging applications built with actix-web.

Use strace or https://mitmproxy.org/ to see what exactly is sent to/from server. It might be some header you are missing in this case.