Hahaha this is so stupidly simple. 
I even used this guide before but I guess I missed that the directory hadn’t been created.
I fixed it in less than a minute by doing this:
sudo mkdir /var/www/munin
sudo chown munin:munin /var/www/munin
and then
sudo service munin-node restart
Works now.
Glad we could get to it… I also created this directory and opened it up (didn’t exist) so that was probably it. … Good job man 
We are configuring munin to use a differently directory so this fix would not apply to Mail-in-a-Box.
I am using a clean install, then the mailinabox script… so it seems like some servers just won’t work without that directory.
I checked again… the directory i created was a actually
/home/user-data/www/default/munin
and
/home/user-data/www/default/admin
It’s possible neither of these directories are correct either… anyway something we both did got it working.
Clean installs
I had the same error when migrating. I could fix it by setting the hostname of the munin-node correctly.
Check what’s the hostname used by the munin-node:
nc 127.0.0.1 4949
This should return something like box.mydomain.com. My returned box.mydomain.com.local, which is wrong. So set it manually:
nano /etc/munin/munin-node.conf
Find
#host_name
And set it to host_name box.mydomain.com
Okay I think this is fixed, see: