Configure SSL with Ajenti & Nginx

How can I install a SSL certificate for a specific website?

I have uploaded the cabundle.crt and certificate.key to /etc/ssl and added this path to the "SSL" tab of the website.

But then Nginx says:

nginx: [emerg] SSL_CTX_use_PrivateKey_file("/etc/ssl/certificate.key") failed (SSL: error:0B080074:x509 certificate

routines:X509_check_private_key:key values mismatch)

nginx: configuration file /etc/nginx/nginx.conf test failed