Yeah this happened on all my munin nodes. And this was actually lucky as the fix was easy for all systems but one. What I had to do to fix this is uninstall munin-node and then reinstall it. I then grabbed the plugins symlinks with:
tar -C / -cvf munin-node-plugins.tar.gz /etc/munin/plugins/
Then I made my config changes and got the node working.
Then I just dumped the tar onto the other munin-node systems with:
tar -C / -xvf munin-node-plugins.tar.gz
And then all was working again. Thank all that is good as the emails to root where filling things up...