The SNMP processors currently fail when stopped & started again because snmpTarget is not set to null and only if it is are the user credentials added to the current USM/Engine.
So what happens is that SNMP4j complains that there is no username.
I'll provide a patch that fixes this.
In addition I'd like to update SNMP4J from the version that's being used 1.x (from 2009) to 2.5.6 from 2017. And I'd love to also do a code cleanup run. I'd prefer to have it all in one patch (it still won't be large) but if really needed I can separate it out into three patches.