Nginx error: no resolver defined to resolve apache_backend_http_#_#_#_#

Symptoms

Your site gives a 502 error on a server running Nginx, and an error similar to the following may be found in the Nginx error log.

2024/10/21 23:35:03 [error] 1299#1299: *236 no resolver defined to resolve apache_backend_https_127_0_0_1, client: 203.0.113.2, server: domain.tld, request: "GET /resource HTTP/1.1", host: "www.domain.tld", referrer: "https://host.domain.tld/

Description

This issue most often occurs when the domain's userdata has the incorrect IP address, and the Nginx configuration was built using that incorrect address.

Workaround

The userdata must be corrected or replaced, and then the Nginx configuration must be rebuilt.

Additional resources

How do I fix missing or corrupted cPanel userdata?

How to restore an account from your server backups

Restore an account from a backup file on the server

How do I configure all sites on the server for ea-nginx?