After restarting a monitored SNMP device (Funkwerk R1200/R3000 router) via SNMPv3, the monitoring stopped working. The device complained about authentification problems.
A tcpdump showed, that the engineTime differs after the restart, and Zabbix is not syncing again, which produces usmStatsUnknownEngineIDs and usmStatsNotInTimeWindows SNMP messages.
Zabbix stops monitoring until restart of zabbix-server and reactivating the affected items.
Further information can be found in post #17 of http://www.zabbix.com/forum/showthread.php?t=11855&page=2