Reverse proxy failing with unknown error

I'm trying to self host plausible analytics. So far I've installed docker, and started the docker containers. I've also ensured nginx is running, has a valid config, allowed ports 80 and 443 through ufw, and configured nginx with the domain: https://plausible.example.com.

My nginx config looks like this:

   server {
    server_name plausible.example.com;

    location / {
        proxy_pass http://localhost:8000;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }

    listen [::]:443 ssl; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/plausible.example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/plausible.example.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 ($host = plausible.example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    server_name plausible.example.com;

    listen 80;
    listen [::]:80;
    return 404; # managed by Certbot 
}

The problem is when I try to visit plausible.example.com I get a 502 bad gateway error. When I check the nginx error logs I get this error:

[error] 935230#935230: *579224 recv() failed (104: Unknown error) while reading response header from upstream

I've tried to find a solution to this error but I haven't been able too. Any help would be appreciated. Thanks.

Edit: I should also add that in the plausible env variables, the host name is: https://plausible.example.com