Question Bad Gateway error (502) on nginx

[MartinH]

I followed 502 Bad Gateway nginx : How to solve? but can not resolve my issue on Obsidian:
My localhost is listening on port 5000 - netstat -aon shows

tcp  0  0 127.0.0.1:5000  0.0.0.0:*  LISTEN

and Apache is up and running - (checked with service apache2 status), Fail2Ban is not installed
nginx Proxy Mode is turned off, and an additional nginx directive is set:

location /api/ {
    proxy_pass http://127.0.0.1:5000;
}

When trying to post (e.g.) example.com/api/login the server responds with Bad Gateway 502.
The log (Websites & domains > Logs) shows

502    GET /api/login HTTP/1.1
261955#0: *1686 upstream prematurely closed connection while reading response header from upstream

Any help in resolving the issue is greatly appreciated.

Further info:
The project is a MERN stack app, consisting of frontend and backend. Both parts are in separate folders under the same vhost-name. The frontend is reachable without any problem. The backend is a node-server (up and running). I tested to open the port 5000 directly and was able to work with it with a post to example.com:5000/api/login. I chose to setup node directly in the backend (and not via the Plesk interface), as I wanted to keep the separation between front- and backend.

 

[MartinH]

Maybe this will help whoever is faced with the same problem:
The upstream error means that there is something wrong in the communication from nginx up to the receiver (in my case nodejs running on localhost).
I was under the (wrong) impression, that the whole site needed to employ SSL and set up node accordingly. That is why I was able to post a https://example.com:5000/api/login from Postman (with the appropiate port opened).
In the production line however I passed all requests through nginx, which was set by my nginx directive to redirect to localhost with http:

location /api/ {
    proxy_pass http://127.0.0.1:5000;
}

Since node was expecting https the two did not work together properly. I changed node to receive http only (require("http") instead of require("https")) and then everything worked smoothly.