Symptoms

User granted the process rights to Domain Users Group. Out of sudden, all the users in Domain Users group lost their rights to start the process right with following error:-
User xxx do not have the rights to start the process
 

Diagnoses

There is a known issue in Blackpearl 4.6.6/7/8 where the Group membership will be removed when there are flooding calls to Active Directory to retrieve user details.
 

Resolution

To resolve this immediately, run this script to force recaching of Identity Group Members:-
Update tIdentity].yIdentity] Set ExpireOn=GetDate(), ContainersExpiredOn=GetDate(), MembersExpireOn=GetDate(), Resolved=0, MembersResolved=0, ContainerResolved=0 Where Name Like '%Domain User%'

To prevent this from happening again, get a coldfix from labs on this.