From time to time SNMPv3 requests (items) just stop delivering data. snmpget from the command line for the same OID works.

I looked a little bit deeper into the packets exchanged between Zabbix server and the network node. It seems that the Zabbix server gets confused the the time since last boot of the device and thus only gets SNMPv3 errors "usmStatsNotInTimeWindows.0"

Packets on the line:
1) x.x.x.x:60674 > y.y.y.y:161: { SNMPv3

{ F=r }

{ USM B=0 T=0 U= } { ScopedPDU E= C= { GetRequest(14) R=1978855259 } } }
No boot and time set (B=0, T=0). This paket is exchanged to get the EngineID from the node.

2) y.y.y.y:161 > x.x.x.x:60674: { SNMPv3 { F= } { USM B=0 T=0 U= }

{ ScopedPDU E= 0x800x000x000x090x030x000x050x730xB70x9D0x40 C=

{ Report(33) R=1978855259 .1.3.6.1.6.3.15.1.1.4.0=914879 }

} }
The node reports its EnginneID. Please note the the note does NOT report boots and time since last boot (B=0, T=0).

3) x.x.x.x:60674 > y.y.y.y:161: { SNMPv3

{ F=ar } { USM B=19 T=3528723 U=xxxx } { ScopedPDU E= 0x800x000x000x090x030x000x050x730xB70x9D0x40 C= { GetRequest(34) R=1978855258 .1.3.6.1.2.1.2.2.1.2.436207616 } } }
The Zabbix server asks the node for the OID in question. Please note that Zabbix suddenly "knows" the nuber of boots and time since last boot of the node (B=19, T=3528723).

4) y.y.y.y:161 > x.x.x.x:60674: { SNMPv3 { F=a } { USM B=19 T=153362 U=xxxx } { ScopedPDU E= 0x800x000x000x090x030x000x050x730xB70x9D0x40 C= { Report(33) R=1978855258 .1.3.6.1.6.3.15.1.1.2.0=144804 } } }
The node reports the usmStatsNotInTimeWindows.0 error (OID 1.3.6.1.6.3.15.1.1.2.0) and reports it REAL time since last boot: T=153362

5) x.x.x.x:60674 > y.y.y.y:161: { SNMPv3 { F=ar }

{ USM B=19 T=3528724 U=xxxx }

{ ScopedPDU E= 0x800x000x000x090x030x000x050x730xB70x9D0x40 C=

{ GetRequest(34) R=1978855258 .1.3.6.1.2.1.2.2.1.2.436207616 }

} }
The Zabbix server is NOT impressed by the error and the correct time, nut sends out its SNMPv3 packet with the wrong time again. Please note the T=3528724, one second more that before.

6) y.y.y.y:161 > x.x.x.x:60674: { SNMPv3

{ F=a }

{ USM B=19 T=153363 U=oper }

{ ScopedPDU E= 0x800x000x000x090x030x000x050x730xB70x9D0x40 C=

{ Report(33) R=1978855258 .1.3.6.1.6.3.15.1.1.2.0=144805 }

} }
Of course the node sends the same error again.