When adding a new cert (or replacement cert) from the command line we received the above error.
The original key was used with openssl and there was no passphrase for the key file.
Simply converting the key (not the cert) from the Netscaler shell worked using the following command:
openssl rsa -in ‘originalkeyfile’ –out ‘newkeyfile’ fixes the file and allows for addition of the certificate and its key to the netscaler
Adding the cert to the netscaler from the command line
add certkey CERTIFICATE_FULL_NAME.cer -cert /nsconfig/ssl/MYCERT_CERT.cer -key /nsconfig/ssl/MYCERT_KEY.key