nginx with multiple certificates, letsencrypt and self-signed

@Sven: Actually, the proposed configuration works. @garethTheRed: yes, you are right, ssl_certificate and ssl_client_certificate are unrelated

Full working configuration.

server {
    listen                  443 ssl;

    server_name             <frontend>;
    include                 snippets/ssl-params.conf;

    ssl_certificate         /etc/nginx/certs/letsencrypt_fullchain.crt;
    ssl_certificate_key     /etc/nginx/certs/letsencrypt.key;

    # Mandatory certificate request setup, self-signed certs
    ssl_client_certificate  /etc/nginx/certs/ca_to_verify_agains_not-not_letsencrypt.crt;
    ssl_crl                 /etc/nginx/certs/ca_to_verify_agains_not-not_letsencrypt.crl;
    ssl_verify_client       on;

    location /secret {
        proxy_pass          http://192.168.122.100:80/secret;
        proxy_redirect      http://192.168.122.100:80/secret https://<frontend>/secret;
        proxy_set_header    X-Real-IP  $remote_addr;
        proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header    X-Forwarded-Proto https;
        proxy_set_header    X-Forwarded-Port 443;
        proxy_set_header    Host $host;
    }
}