That error means that the key file does not match the certificate nginx is using.
Running these commands should output the same hex numbers:
openssl rsa -noout -modulus -in ssl.key | openssl md5
openssl x509 -noout -modulus -in ssl.crt | openssl md5
(Replace ssl.key and ssl.crt with the path to the key and certificate file.) You will probably not get a match.
My guess would be that the certificate file is a bundle and has the server and CA keys in the wrong order for nginx.
I.e., your certificate files has:
Nginx needs to see the first certificate in the chain (your site certificate) before the second one. Editing the file to swap the order should fix it.