When I try to add user/group using SharePoint OM or Power Shell script I'm getting the following error. In this example S-9-9044-V is an AD Group.
The user does not exist or is not unique
$web = get-spweb "http://sharepoint/Year9.SCI"
$group = $web.sitegroups["Year9.SCI Students"]
$user = $web.EnsureUser("school\S-9-9044-V")
$group.AddUser($user)
I also noticed that this happens only for few AD groups and I'm able to add other users/groups successfully. Some suggested to reconfigure object cache user accounts but that doesn't help as I'm able to add other AD groups using the same script. http://technet.microsoft.com/en-us/library/ff758656.aspx
Any suggestions?