Nagios killed all web sites created by virtualmin

I have a bunch of domains running on a Linux box. They are managed by VirtualMin. Each site has its own user name, group and home folder.
For example, site www.infolexllc.com under VirtualMin has user name and groups infolex as well as home folder /home/infolex.
The document root of the site is /home/infolex/public_html.
Everything was working great until I installed Nagios, hoping it could help me monitor status of my server.
But after Nagios was installed, all my sites on this server became unaccessible. when I pointed my browser to:
http://www.infolexllc.com
I got 403 error saying there's no permission to access the site.

I remember I ran into similar issue on a client's server before. So I found log I wrote down about two years ago. Looks like Nagios somehow removed user apache from group apache. Therefore, Apache server lost access right to website's document root which was owned by apache.apache.

So I guess similar thing must happened to my server also. I checked owner of /home/infolex/public_html, it's infolex.infolex.

I check groups apache belongs to, they are:

apache, nagois, nagiocmd

So I added apache back top group infolex by:

/usr/sbin/usermod -a -G infolex apache

I did the same to all other virtual hosts on the same server.

Then restart Apache. Everything is back to work!

I'm still having problem with Nagois itself so far. Will post more if I ran into anything worth to share.