This is one possible reason for the error "Bad trap packet received from Node with IP x.x.x.x. Error description: Unknown user and engine. Packet discarded."
I ran into this issue not too long ago. All of our Cisco routers & switches are running SNMPv3 and I was receiving the error "Bad trap packet received from Node with IP x.x.x.x. Error description: Unknown user and engine. Packet discarded." that was showing up in the Events window. After troubleshooting that SNMPv3 was configured correctly on the routers and switches. I noticed that the IP address in the error message was actually assigned to interfaces on the routers and that the traps were being sent from the interfaces. Since the SNMPv3 credentials are used by the router and not the interfaces I needed to find a way to source all traps from the router's Loopback address themselves. The easiest way to do this was to enter the following line within the SNMP portion of the router configuration. "snmp-server trap-source Loopbackxxxx" Where xxxx is the loopback # that the router's IP address is assigned to.