Looks like the root certificate is missing here. ASA does not know the identity of Digicert and hence can't trust it. I am assuming that you did not generate CSR from ASA. You would ideally need to upload the identity and CA certificate separately or PKCS12 format which consists of identity cert, CA and private key bundled together.
Did you follow one of the methods defined in below link:
http://www.cisco.com/c/en/us/support/docs/security-vpn/public-key-infrastructure-pki/200339-Configure-ASA-SSL-Digital-Certificate-I.html
-AJ