Starting from Windows plesk 7 till plesk 11 one of the most repeating issue that a hoster will often face is
HTTP Error 401.3 - Unauthorized Error
You do not have permission to view this directory or page because of the access control list (ACL) configuration or encryption settings for this resource on the Web server.
One most likely solutionn to this, independent of which plesk version or IIS you are using, is to grant permissions to IUSR_domainuser user to the C:\Inetpub\vhosts\domainname.com directory
Replace domainuser with the real domain user, this is the main ftp user of the site. In plesk 11 domain user is visible on main subscription page as systemuser.