What do these messages mean and what can be done about them?
When the SNMP Fetcher daemon restarts on an Airwave server it sends an initial poll to the controller where EngineBoots and EngineTime are equal to 0:
[ msgAuthoritativeEngineBoots: 0
msgAuthoritativeEngineTime: 0 ]
These values are supposed to be synced with the controller's side of the USM in SNMPv3.
However, this initial poll is done to sync with the controller and thus is considered to be an out of sync messages on the controller's side. Once this sync is done no further errors are generated unless the SNMPv3 USM becomes out of sync again.
If they are an annoyance due to filling the logs a lower logging level than Warnings can be selected on the controller under Configuration->Logging->Levels Tab. [Choosing a lower logging level is just a workaround and not preferred recommendation]
If the messages are occurring frequently (at least once a minute) it can indicate that Airwave's SNMP Fetcher daemon is restarting often. Also, it can mean that the clock on the Airwave server and the SNMPEngineTime on the controller aren't keeping time properly.
These messages can be ignored in almost every case, see the resolution section above if these messages are being seen at least once a minute or you would not like them to be logged.