The reason this is happening is because Spectrum's current SNMP stack implementation is unable to reset cached SNMPv3 USM user information, so a SpectroSERVER restart is required to reset this information. This will be fixed in 9.1 Hotfix 11 with the following release note entry:
Changing configuration of SNMPv3 devices makes them unaccessable without
restart of SpectroServer.
Capability to dynamically reset device's SNMPv3 parameters (via
delete/reset/discover model) without restarting SpectroServer will be added to
resolve this problem.
Cannot model SNMPv3 devices or lose contact to v3 devices in Spectrum
Causes of this problem:
Receiving 'Management Agent Lost' alarms or are unable to model SNMPv3 devices. If this is happening and your devices have been recently rebooted or upgraded, you will have to restart your SpectroSERVER and this should clear the alarms or you should be able to now successfully model these devices in Spectrum.
(Legacy KB ID CNC TS34165 )