I opened a code-server installation accessible at vscode.jgalec.me with Nginx as a reverse proxy, I navigated to the subdomain and everything worked, the code-server login prompt, etc, but I wanted to secure it applying a free Let’s Encrypt TLS certificate, so I run the sudo certbot --nginx -d code-server.your-domain to request certificate and it was successfully enabled
Thanks for confirming that you were able to find and use a solution. This can be so helpful for future helpseekers who may be having the same or similar issues you're having.
It's actually fairly common with some cloud hosters.. I don't know anything about those cloud stuff, but I do know some (or just one, I dunno) have two firewalls which can be configured separately..
Not sure! If I were you, I'd certainly mention it.. Although it might be even better to compile some sort of "Most common reasons why you get the following error message" page with per possible error message a list of common reasons why.