ISA Server does not have the security permissions required to check user group membership. As a result, ISA Server is unable to evaluate rules that are applied to user sets that contain Windows users and groups.
ISA Server must have additional permissions in order to access user group information. This is controlled via the TokenGroupsGlobalAndUniversal security attribute.
On a Windows 2003 domain, add the ISA Server computer accounts to the Windows Authorization Access Group.
On a Windows 2000 domain, since the Windows Authorization Access Group does not exist you should create a domain local group for this purpose, and add the ISA Server computer accounts to that group. Allow this group to access the TokenGroupsGlobalAndUniversal attribute on user objects, on computer objects, and on iNetOrgPerson objects.