I think you need to enable SSL to be able to modify the AD. To enable SSL you have to import the AD certificate in the vCO server. To do that you can use the OpenSSL tool:
openssl s_client -connect host.host:9999
Replace the host and port with the URL shown on the AD Plugin configuration page. Then copy from -----BEGIN CERTIFICATE----- to -----END CERTIFICATE----- including this lines and create a new file. Import the new file to vCO.