How do I fix a 502 bad gateway error combined with a 302 Nginx connection refused error?

My hosted node app stopped working with a 502 Bad Gateway Error and I noticed 302 connection errors when I typed in sudo tail -30 /var/log/nginx/error.log

api = the subdomain I’ve put the node app on

2021/03/10 21:01:44 [error] 283085#283085: *302 connect() failed (111: Connection refused) while connecting to upstream, client: 68.49.148.244, server: api.domain.com, request: "GET /favicon.ico HTTP/1.1", upstream: "http://127.0.0.1:3001/favicon.ico", host: "api.domain.com", referrer: "https://api.domain.com/db/test"

sudo nginx -t returned saying that my nginx configuration is ok

this is the file in nginx/sites-available (api is standing in for the subdomain I’ve put the node app on):

server_name api.domain.com www.api.domain.com;
location / {
proxy_pass http://localhost:3001;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
}

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/api.domain.com/fullchain.pem; # managed by Certb>
    ssl_certificate_key /etc/letsencrypt/live/api.domain.com/privkey.pem; # managed by Cer>
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot


}

This didn’t start happening until yesterday. Could someone help me figure out what’s gone wrong, please?