I have exact same warning from the daily mail sent from the box:
Subject: Cron <root@box> (cd /root/mailinabox && management/daily_tasks.sh)
WARN: uid is 0 but '/' is owned by 107
I have installed MIB a few days ago, on a new bare Linode with Ubuntu 14.04.
I have not done any chown. Also, user 107 is the Name Server Daemon (NSD).
It seems a bug related to the install.