Write operation failed

The following error appears in the IdB logs on an attempted full import from the employee OU:

Handling of LDAP search request from user mimuser on connection 127.0.0.1:50264 targeting OU=employees,DC=IdentityBroker with a scope of WholeSubtree failed with error "The write operation failed, see inner exception.". Duration: 00:01:25.7969304.

MIM MA Import fails with the standard "stopped-extension-dll-exception", but the event log reports a time-out:

The extensible extension returned an unsupported error.
 The stack trace is:
 
 "Unify.Product.IdentityBroker.LdapOperationException: Error during processing of SearchRequest targetting OU=employees,DC=IdentityBroker: Operation timed out while waiting for message queue with id of 13. ---> System.OperationCanceledException: Operation timed out while waiting for message queue with id of 13.
   at Unify.Product.IdentityBroker.LdapConnection.GetMessage(Int32 messageId)
   at Unify.Product.IdentityBroker.SearchRequest.Send(Func`2 send, Func`2 recv)
   at Unify.Product.IdentityBroker.LdapConnection.SendRequest(ILdapRequest request)
   --- End of inner exception stack trace ---
   at Unify.Product.IdentityBroker.LdapConnection.SendRequest(ILdapRequest request)
   at Unify.Product.IdentityBroker.LdapConnectionProxy.d__8.MoveNext()
   at Unify.Product.IdentityBroker.ImportProxy.d__29.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Linq.Enumerable.d__17`2.MoveNext()
   at System.Linq.Enumerable.d__17`2.MoveNext()
   at System.Linq.Enumerable.d__17`2.MoveNext()
   at Unify.Product.IdentityBroker.ExtensionMethods.Take[TSource](IEnumerator`1 source, Int32 count, IList`1& items)
   at Unify.Product.IdentityBroker.ExtensionMethods.d__3`1.MoveNext()
   at Unify.Product.IdentityBroker.ImportProxy.Import(GetImportEntriesRunStep importRunStep)
   at Unify.Product.IdentityBroker.UnifyLdapConnector.GetImportEntries(GetImportEntriesRunStep importRunStep)
Forefront Identity Manager 4.4.1459.0"