I too had this issue when migrating from S300 to S690. It was fixed after removing the below lines and contents from the backup XML.
<prox_config_secure_auth_cert_name></prox_config_secure_auth_cert_name><prox_config_secure_auth_cert></prox_config_secure_auth_cert>
<prox_config_secure_auth_key></prox_config_secure_auth_key>