Slaves were found to be responding REFUSED to queries for zones they should have been authoritative for.  There was no indication in the logs that they had dropped their catalogs.  A stop/start on both slaves recovered their state, but it was noted during startup that they both reported syntax errors in the slave files for zones in the catalog.

There was next to nothing in the logs to explain why the slaves stopped serving these zones.

Could this be due to a misconfiguration, or something else?