Unable to set the private key: Probably, the private key format is invalid.

Hi,
I have bought a SSL certificate, I have try to install on server with plesk 12.5.

I follow this steps:

1. I have generated the .key and .csr files
2. I have send the .csr to the certification authority
3. The certification autority has sent me the Cert and the intermeditate Cert.
4. In plesk I go mydomain -> SSL Certificates -> Add SSL certificate

On bottom of page in section "Upload the certificate as text"

• in the section...

Unable to set the private key: Probably, the private key format is invalid.