nginx "connect() failed (111: Unknown error) while connecting to upstream" error

I have an nginx server that’s giving me an error that I can’t find any information on. The server is up and running and I believe the underlying site (a Flask app) is also running, but clients receive a generic 500 error. When I look at /var/log/nginx/error.log, I see this:

2022/09/04 04:59:52 [error] 1523#1523: *1 connect() failed (111: Unknown error) while connecting to upstream, client: [clients ip], server: mysite.com, request: "GET / HTTP/1.1", upstream: "http://[::1]:8000/", host: "mysite.com"

I have no idea what to do with this. I’ve searched around and can’t find any info on how to dig in deeper or where to look to resolve the issue.

How I got into this state

I’m running the server on Ubuntu on a Linode and wanted to upgrade the OS. To do so, I spun up a new Linode with the latest version of Ubuntu (22.04) and recreated the server from scratch. After getting everything installed, I swapped the IP addresses of the old and new servers to avoid having to redo the DNS records. I’m using CertBot to manage https certificates, so I ran that again and then restarted everything.

At this point, nginx was up and running and happy with my config. Everything looks the same as it did on the old server. But every request is a 500 with the error above.

Any help / pointers to get me back to a working state is very much appreciated!

EDIT: nginx config

server {
    server_name = www.mysite.com;
    return 301 $scheme://mysite.com$request_uri;

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/mysite.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/mysite.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}

server {
    server_name mysite.com;

    location ^~ /static/ {
        include /etc/nginx/mime.types;
        root  /home/my_username/path/to/code/;
    }

    location / {
        proxy_pass http://localhost:8000;
        include /etc/nginx/proxy_params;
        proxy_redirect off;
        proxy_read_timeout 300s;
    }

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/mysite.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/mysite.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot


}
server {
    if ($host = www.mysite.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot
    if ($host = mysite.com) {
        return 301 https://$host$request_uri;
    } # Manually added by me

    server_name = mysite.com;
    listen 80;
    return 404; # managed by Certbot
}