Hello,

Regarding the correct IPv6 Nginx configuration, I could suggest using the Nginx Config Tool which will generate the configuration file for you:

https://www.digitalocean.com/community/tools/nginx

Regarding the 502 error, I would recommend following these steps in case that you are having any problems with your nginx server and you are unsure on what the problem is:

• Check if nginx is running:

systemctl status nginx

If nginx is running you should see something like this:

● nginx.service - The nginx HTTP Server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: active (running) since Tue 2019-11-19 09:37:46 UTC; 2 days ago
     Docs: https://httpd.nginx.org/docs/2.4/

If nginx is not running then the output would look like this:

● nginx.service - The nginx HTTP Server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Fri 2019-11-22 08:41:01 UTC; 39s ago
     Docs: https://httpd.nginx.org/docs/2.4/

• If nginx is not running you could start it with:

systemctl start nginx

Then check the status agian and make sure that nginx remains running.

• If nginx did not start after a reboot, you could enable it so that it starts after the next reboot:

systemctl enable nginx

• Check your nginx config syntax:

nginx -t

If you get an error, you would need to fix that problem and then you could restart nginx:

systemctl restart nginx

• If you get Syntax OK when running nginx -t then your confiruation is correct, so I would recommend checking your error logs:

tail -f /var/log/nginx/error.log

• Check the permissions of the files and folders in your document root:

Find the user that your nginx service is running as:

ps auxf | grep nginx

If you are using Ubuntu, the user should be www-data, so you would need to make sure that your files and folders are owned by that user, so nginx could read and write to those files:

chown -R www-data:www-data /var/www/yourdomain.com

• Check if nginx is binding to the default ports:

netstat -plant | grep '80\|443'

• Check if ufw allows TCP connections on port 80 and 443:

ufw status

If this is the case, you can follow the steps from this article here on how to configure your ufw:

https://www.digitalocean.com/community/tutorials/how-to-set-up-a-firewall-with-ufw-on-ubuntu-18-04

That is pretty much it, with all of the above information you should be able to narrow down the problem.

For more information I would suggest checking out this article here:

https://www.digitalocean.com/community/tutorials/how-to-troubleshoot-common-site-issues-on-a-linux-server

And here is also a quick video demo on how to do that as well:

Hope that this helps!
Regards,
Bobby
Source: How to Troubleshoot Common Nginx Issues on Linux Server?