Mod security will increase the security of your server, but sometimes it's blocking legitimate traffic. If you want to disable it for a single domain on your Plesk server, this is what you need to do:

1. Create a vhost.conf file and place it in the following directory:
   
   /var/www/vhosts/domainname/conf
   
   
2. It should include the following:
   
   <IfModule mod_security2.c>
   SecRuleEngine Off
   </IfModule>
   
   
3. Reconfigure Apache:
   
   /usr/local/psa/admin/bin/websrvmng -u --vhost-name=domainname.com
   
   
4. Restart Apache:
   
   service httpd restart

That's it, Mod Security is now disabled for this single domainname...