Additionally, the following event ID is logged:
Source: LS User Replicator
Event ID: 30061
Task Category: LS User Replicator
Failed to synchronize remote domain contoso.com due to missing data privacy boundary for local or remote domain. Source of replication is: AddressBookReplication.
This is usually caused when privacy boundary is not associated with local or remote site.
<Provider Name="LS User Replicator" />
Lync Server is deployed in Child domain or server / pool name naming convention has child and parent domain relationship.
This forces the user replicator to query all the available domains.