How do I correct a 502 Bad Gateway, nginx/1.18.0 error?

The 502 Bad Gateway error began after I updated some application code and did a restart on nginx on my linode but made no changes to nginx or gunicorn config files. Prior to this, it had been running for over a year without a problem.

I'm running a flask application.

nginx error.log:
2024/04/12 07:11:52 [warn] 91781#91781: conflicting server name "170.187.207.132" on 0.0.0.0:80, ignored
2024/04/12 07:11:52 [warn] 91781#91781: conflicting server name "talk-time.org" on 0.0.0.0:80, ignored
2024/04/12 07:11:52 [warn] 91781#91781: conflicting server name "www.talk-time.org" on 0.0.0.0:80, ignored
2024/04/12 07:11:52 [warn] 91782#91782: conflicting server name "170.187.207.132" on 0.0.0.0:80, ignored
2024/04/12 07:11:52 [warn] 91782#91782: conflicting server name "talk-time.org" on 0.0.0.0:80, ignored
2024/04/12 07:11:52 [warn] 91782#91782: conflicting server name "www.talk-time.org" on 0.0.0.0:80, ignored
2024/04/12 07:12:23 [error] 91784#91784: *1 connect() failed (111: Unknown error) while connecting to upstream, client: 24.16.175.241, server: 170.187.207.132, request: "GET / HTTP/1.1", upstream: "http://[::1]:8000/", host: "talk-time.org"
2024/04/12 07:12:25 [error] 91784#91784: *1 recv() failed (104: Unknown error) while reading response header from upstream, client: 24.16.175.241, server: 170.187.207.132, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:8000/", host: "talk-time.org"
2024/04/12 07:12:25 [error] 91784#91784: *1 no live upstreams while connecting to upstream, client: 24.16.175.241, server: 170.187.207.132, request: "GET /favicon.ico HTTP/1.1", upstream: "http://ttproxy/favicon.ico", host: "talk-time.org", referrer: "https://talk-time.org/"

The pertinent information seems to be "no live upstreams while connecting to upstream" but I don't know where to go from there.

Here is /etc/nginx/sites-enabled/app.conf:

upstream ttproxy {
server localhost:8000;
}

server {
listen 80;
server_name 170.187.207.132;
server_name talk-time.org;
server_name www.talk-time.org;

location / {
    proxy_pass http://ttproxy;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $host;
    proxy_redirect off;
    proxy_ssl_server_name on;
}

listen 443 ssl; # managed by Certbot
ssl_certificate /etc/letsencrypt/live/talk-time.org/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/talk-time.org/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 ($host = www.talk-time.org) {
return 301 https://$host$request_uri;
} # managed by Certbot

if ($host = talk-time.org) {
    return 301 https://$host$request_uri;
} # managed by Certbot

if ($host = 170.187.207.132) {
    return 301 https://$host$request_uri;
}

listen 80;
server_name 170.187.207.132;
server_name talk-time.org;
server_name www.talk-time.org;
return 404; # managed by Certbot

location / {
    proxy_pass http://ttproxy;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $host;
    proxy_redirect off;
    proxy_ssl_server_name on;
}

}

I have applied every fix that I have found in the community posts and anything I have been able to find in stackoverflow, etc.but they have made no difference.

Any help would be appreciated.