You could try this

this config snippet is assuming thet you've already got the TLS cert/pem file for lemmy.world elsewhere in your nginx.config

http {
  server {
    listen 80;
    listen 443 ssl;
    server_name lemmy.world;

    if ($scheme = "http") {
        return 307 https://$host$request_uri;
    }

    location / {
        proxy_pass http://lemmy-ui:1234;
        proxy_set_header Host $host;
    }
}

If you get redirected to lemmy.world:1234, then add absolute_redirect off; in the 'server' block

Last thing - 307 is a temporary redirect, you might to change it to a permanent one once you've confirmed it's working as intended

Can we get an error log? If no, are you seeing any timeouts in there?

You might want to add the secure port (:443) in your redirect. Otherwise it might be trying to load https on port 80 still, which can’t work.

• http: port 80
• https: port 443

Notes:

• just a guess. I haven’t looked at an nginx config in a while
• make sure to try on multiple browsers as they all don’t behave the same way