I've encountered this a bit on some modules (specifically running older versions) but even on 8.1, and particularly on some VSAs I've had the SNMP agent stop responding.
If I disable/enable the SNMP agent on the module it fixes the problem... Any ideas?
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Any previous known issues that would seem to fall under this description should be fixed in 8.1. Support would need to see logs to tell if there is still an issue for 8.1. You should open a ticket.