Issue: Users are not indexed in the database that should be.
Lync Server 2013 Address Book Server will generate error event 21054 once every day when performing daily maintenance. The error is also generated every time an administrator runs the Update-csAddressBook cmdlet, even when the update is successful. However, this error event can safely be ignored when the update is successful.
Responsible / Workaround:
If you see event 21054 on daily basis from Lync FE server. Then you must check and test address book services.
How you can test address book services pool and single user?
Test-CsAddressBookService -TargetFqdn lyncpool13.mydomain.com
Test-CsAddressBookService -TargetUri https://lyncpool13.mydomain.com/abs/handler -UserSipAddress “sip:email@example.com”
When you encounter this error event, run the following cmdlet:
Debug-csAddressBookReplication -Poolfqdn lyncpool13.mydomain.com
If the cmdlet reports that there are no unindexed or abandoned objects, the error event 21054 can be safely ignored.
Additionally, the Key Health Indicator (KHI) "Address Book Users Correctly Indexed" should be turned off in System Center Operations Manager.
Referred: Release notes for Lync Server 2013 on Technet.