Microsoft Knowledge Base Article 870695 contains:
On your Microsoft Windows Server 2003-based domain controller, the following Error event is logged in the Directory Service event log:
Source: NTDS Replication
Event ID: 1988
User: NT AUTHORITY\ANONYMOUS LOGON
The local domain controller has attempted to replicate the following object from the following source domain controller. This object is not present on the local domain controller because it may have been deleted and already garbage collected.
Source domain controller:
Replication will not continue with the source domain controller until the situation has been resolved.