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HTTP_STATUS_CERTIFICATE_REQUIRED
It appears in mutual TLS configurations where the server requires a certificate from the connecting client. A normal browser certificate prompt may be absent if no eligible client certificate is installed or selected.
Check the client certificate store, private-key availability, and the NGINX client-certificate verification policy. Supplying only username/password credentials does not satisfy mutual TLS.
This NGINX-specific result points to mutual TLS configuration rather than ordinary HTTP authentication. Verify that the client actually sends a certificate when requested and that the selected virtual host requires one. If the client owns several certificates, also confirm that it chooses a certificate acceptable to the server certificate-authority list.
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