All the Lync server 2013 environments I’ve deployed so far is giving Event ID 21054 from LS Address Book Server once a day. It also happens when you run Update-CsAdressBook.
I wondered what it was, and found it in the release notes for Lync Server 2013. Turns out that the issue can be safely ignored as long as the update is successful. You can also check the replication by running
Debug-csAddressBookReplication -Poolfqdn <Pool FQDN for which the event was generated>
Also, if using SCOM, the Key Health Indicator (KHI) “Address Book Users Correctly Indexed” should be turned off for the pool.
Lync Server 2013 release notes: