Action required! An account-identifier (RID) pool was allocated to this domain controller. The pool value indicates this ...

Action required! An account-identifier (RID) pool was allocated to this domain controller. The pool value indicates this domain has consumed a considerable portion of the total available account-identifiers. 

A protection mechanism will be activated when the domain reaches the following threshold of total available account-identifiers remaining: %1.  The protection mechanism prevents the allocation of account-identifier (RID) pools needed to allow existing DCs to create additional users, computers and groups, or promote new DCs into the domain. The mechanism will remain active until the Administrator manually re-enables account-identifier allocation on the RID master domain controller. 

See http://go.microsoft.com/fwlink/?LinkId=228610 for more information.