I tried firmwares from 1.03 to 1.05 final (also tried both 1.0 & 1.01 ADS module) and still couldn't make this NAS join the AD, i just get the "failed" message. I can see in the Event Viewer\Security logs on Kerberos with no failure and the NAS is added on the Active Directory Users and Computers.
But if i put a username that doesn't exist in the AD i get the "user not exist" message, which i think means that the NAS is trying to authenticate/communicate with the AD server.