After changing from rhel4-cpanel to a debian 5 lenny 64 bit server, I did some configs. I want to install nagios and munin for monitoring the box as recommended. It is a debian 5 root server with lamp(linux + apache 2 + php 5 + mysql 5) and webmin preinstalled. After first login I have installed virtualmin gpl module and migrated some cpanel-backup files. And than Nagios but after nagios I got 403 error of apache on all websites. Checking .htaccess, httpd.conf, chmod and chown did not help.
The problem is: nagios removes apache's default user (www-data) from webhosting user groups. Changes effect /etc/group. To fix it edit /etc/group with a text editor and add www-data to every webhosting user.