Nginx running four sites, one unresponsive [non webdev.]

Question

A couple of years ago I set up four websites on a nginx server, recently I ran into SQL troubles and decided to rebuild site 1 which ran the SQL backend. However, ever since I am running into a persistent 502 voor site 1 which I can't seem to get a hold of.

Initially I was running into port issues but this does not seems to be fixed after running through; nginx - nginx: [emerg] bind() to [::]:80 failed (98: Address already in use)

At this point I am stil stuck with a: connect() failed (111: Connection refused) while connecting to upstream, client: 01.01.01.100, server: www.site1.com, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:5001/", host: "site1.com" in the nginx error.log.

Could someone give me a push in the right direction?:(

I don't have a webdev background, my focus is on scientific research through Python and I like to play around with HTML/CSS etc. Server management isn't my cup of tea I'm afraid :(.

Edit 1;

netstat -ln4t displays:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 127.0.0.1:5002          0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:5003          0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:5004          0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:443             0.0.0.0:*               LISTEN

Am I correct in thinking this means :5001 is not up and running?

Edit 2:

Alright, next step. Nginx calls on four seperate Gunicorn sockets (@ /etc/systemd/system) , one for each site, which in turn calls on individual Django backends (site 1 socket is comparable with site 2 socket).

Running:

sudo systemctl status site1.gunicorn

Results in:

   Active: failed (Result: exit-code) since Sun 2022-03-20 11:23:40 UTC; 
 *techbabble*
Main PID: 23301 (code=exited, status=1/FAILURE)
*techbabble*
Mar 20 11:23:40 site1.com systemd[1]: Failed to start Gunicorn daemon

It seems I am onto something!

1. sudo systemctl deamon-reload
2. sudo systemctl restart site1.gunicorn
3. sudo systemctl status site1.gunicorn

Again, a step further, the socket (?) is up! Through the 502 is still persisting, turning off and on nginx didn't solve this.