Failed to update the list of domains in this forest to synchronize. User Replicator cannot synchronize any users without ...

Failed to update the list of domains in this forest to synchronize. User Replicator cannot synchronize any users without knowing which domains to synchronize from.

Cause: This failure occurred because DsCrackNames failed to resolve one of the domain partitions to a valid FQDN.  User Replicator doesn't have a list from a previous cycle, and cannot go any further.  DsCrackNames typically only fail in Lync Server if memory is low on this machine.
Resolution:
If memory is not low and this error continues to occur, the computer needs to be rebooted.