Since you are using SNMP v3, check the engineID string.
Also from the man page of snmpd.conf :
[quote]
| engineID STRING | |
| specifies that the engineID should be built from the given text | |
| STRING. |
| engineIDType 1|2|3 | |
| specifies that the engineID should be built from the IPv4 | |
| address (1), IPv6 address (2) or MAC address (3). Note that | |
| changing the IP address (or switching the network interface | |
| card) may cause problems. |
[/quote]