Happened when connecting to upstream server using the self-signed certificate. Workaround can be to set proxy_ssl_verify nginx directive to off;

Better option is to add the correct certificates in the file pointed to by proxy_ssl_trusted_certificate directive. The file should contain in order, server.crt (and probably any intermediary cert if you have it) then rootCA, e.g.

/etc/ssl/certs/trusted_ca_cert.crt

-----BEGIN CERTIFICATE-----
...
server.crt data
...
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
...
rootCA cert data
...
-----END CERTIFICATE-----

• nginx/1.23.0

• Proxy pass nginx connection using self-signed certificates
• Proxy non-http traffic through nginx
• Example of redirect and rewrites
• Avoid upstream not found in nginx when using only one upstream server