Problem getting started with SNMP

[Colin Law]

I am trying to learn about SNMP. I have two 20.04 machines and am
attempting to follow this tutorial [1] (which admittedly is for 18.04,
so maybe there is some difference there).  I am setting one up as the
manager and one as the agent.

All goes well until I try to change to password of the newly created
demo user with the command (on the manager)

snmpusm -u demo -l authPriv -a MD5 -x DES -A temp_password -X
temp_password agent_server_ip_address passwd temp_password
new_password

with the appropriate IP for the agent machine.  The response is
Error in packet.
Reason: wrongLength (The set value has an illegal length from what the
agent expects)
Failed object: SNMP-USER-BASED-SM-MIB::usmUserOwnPrivKeyChange.".....)D.'..m`...."."demo"

In fact I get the same error with the original bootstrap user if I try
to change his pwd so there is something pretty basic wrong somewhere.
I have looked for alternative tutorials that might give a clue and
struggled through assorted man pages but can't work out what is going
wrong so any help would be much appreciated.

Colin

[1] https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-an-snmp-daemon-and-client-on-ubuntu-18-04