syntax error: unknown user 'munin' in statoverride file

Solution 1:

Take a look in

/var/lib/dpkg/statoverride

if there is any line

root munin 750 /etc/munin/plugin-conf.d

Try:

dpkg-statoverride --remove /etc/munin/plugin-conf.d

Solution 2:

I noticed that not everything was removed from the installation so manually removed the munin web directory and also removed the munin user-name and group from the sever.

Don't do that! Because you didn't purged properly the users stored in /etc/passwd file, which dpkg queries to compare with the statoverride file, do not match up and dpkg is throwing a warning. Instead purge the package and dpkg will remove all the directories and users the package created, without the user having to dirt their hands in it.

sudo apt-get purge package

This should do it.