[AGENT++] Error message: Unknown pdu handlers

[Robert Voigt]

I'm getting the error message 
SNMPv3: Unknown pdu handlers (-1415) 
in a manager, when it's sending snmp gets or sets periodically. The
error occurs every few seconds, then all is normal for a few seconds. 

The strangest thing is, I get the error only when agent and manager run
on the same host, one particular Fedora Core 4 machine. If I run the
same manager remotely to send requests to that agent, all is ok. If I
run that agent somewhere else and run the manager on that machine, all
is ok too.

What does that error message mean and does anyone have ideas where I
could look?