As we see the sysname is without the domain-name. Do I have to rename all my boxes to include domain-name?
No. The check which is being done here is if the DNS name of the IP (sysname field in netbox table) starts with the collected sysname from the device.
That said, it does seem a bit strange that the above will give a dnsMismatch error. The string "volda-XXX.hivolda.no" does indeed start with "volda-XXX". Do you get this error only for this particular device or is it a more general problem? There could potentially be a whitespace issue which throws off the check, but that is at least very easy to fix (I can tell you which line to change in the source code if you want to try).
-- Kristian