Could not complete synchronization on link due to a converging join error

The follow error is appearing in my UNIFYBroker/Plus log.  Could you tell me more about what it means?

Request to sync changes on link failed.
Request to sync changes on link Employees > AD Users (6410cee2-8159-4cc3-89d6-0a3cc3d46fdb) in direction outgoing failed with message Could not complete synchronization on link '6410cee2-8159-4cc3-89d6-0a3cc3d46fdb' due to a converging join error.
First source entity id: cb04dfa0-66e9-46fd-862d-54512e11f2c3
Second source entity id: fb3ee77f-5370-44e7-b1ce-0b01c39c0f88
Offending target entity id: 4ffd33d1-ec16-464d-90cc-ab0fe7d7b93a [Count:6492]. Duration: 00:00:02.2246402
Error details:
System.Exception: Could not complete synchronization on link '6410cee2-8159-4cc3-89d6-0a3cc3d46fdb' due to a converging join error.
First source entity id: cb04dfa0-66e9-46fd-862d-54512e11f2c3
Second source entity id: fb3ee77f-5370-44e7-b1ce-0b01c39c0f88
Offending target entity id: 4ffd33d1-ec16-464d-90cc-ab0fe7d7b93a
at Unify.Product.Plus.LinkSynchronizer`2.JoinAndMap(IEnumerable`1 filterResult, IDictionary`2 changesDict)
at Unify.Product.Plus.Link.SynchronizeChanges[TSourceEntity,TTargetEntity](IEnumerable`1 changes, IEnumerable`1 syncTasks, Func`1 getTargetContextAccessor, IConnectionsContext connectionContext, ISynchronizationHelper`2 helper, IProvisioner`2 provisioner)
at Unify.Product.Plus.Link.SynchronizeAdapterChanges(IEnumerable`1 changes)
at Unify.Product.Plus.LinkNotifierDecorator.<>c__DisplayClass42_0.<SynchronizeAdapterChanges>b__0()
at Unify.Framework.Notification.NotifierDecoratorBase.Notify[TResult](ITaskNotificationFactory notificationFactory, Func`1 function)
at Unify.Product.Plus.LinkNotifierDecorator.SynchronizeAdapterChanges(IEnumerable`1 changes)
at Unify.Product.Plus.LinkAuditingDecorator.SynchronizeAdapterChanges(IEnumerable`1 changes)
at Unify.Product.Plus.AdapterToLockerSynchronizationJob.RunBase()
at Unify.Product.Plus.SynchronizationJobExecutor.<ThreadAction>d__8.MoveNext()