One of the many reasons why you have a 403 error on your website when using Plesk and tweaking your server (updates for exemple that can modify your Apache2 configuration, it happened to me when installing Landscape) might be that you need to add the APACHE_RUN_USER (usually www-data but make sure by looking for it into /etc/apache2/envvars) into the psaserv group (1).
vim /etc/apache2/envvars
/APACHE_RUN_USER # to be run under Vim
# The /{search} command in Vim is the search command. Note the user, it should be www-data.
sudo usermod -a -G psaserv www-data # Adding the user back into the psaserv group
Using CentOS, the user must be “apache”.
sudo usermod -a -G psaserv apache
tomsihap 