After getting our new TFS 2012 server built out I found myself unable to grant permissions to the Tfs_Analysis database in SQL Analysis services. When attempting to add a new user of the local Active Directory domain to the TfsWarehouseDataReader role I received the above error.
I found that the role membership contained a user that was unresolvable via Active Directory and showed up as a SID only (no domain\name). Removing that unresolved user allowed me to add new groups to the role from the local AD.
It would appear from this that the logic is revalidating all of the users and groups that are listed in the membership role list when adding/removing members.
