Nginx error 111

How do I troubleshoot the following error in my nginx error log file:

2019/12/31 00:25:13 [error] 1182#1182: *706200 connect() failed (111: Connection refused) while connecting to upstream, client:, server:, request: "GET /wp-login.php?action=register HTTP/1.1", upstream: "", host: ""

Google suggested the loopback address might be related to the problem, would appreciate if anyone could give some troubleshooting tips.