On Plesk 9.x, Plesk created "users" for each web/domain which are the user accounts that access web resources like php and HTML files.
Under Plesk 9.x these users were called: PLESK IIS User (hostname\IUSR_someusername)
This "user" under 9.x had the "full control" to do things like modify config.inc.php.
Plesk 12 (at least) changed this model an went with a 'least privilege' model. Additionally, they renamed this user so as to make it a bit tougher to locate.
Under plesk 12 this user is: IWPG_someusername. By default this user only has 'read' permission. By granting this user 'full control' on this file UBB Threads Control Panel can now modify config.inc.php to do things like "close the message boards".
Now, with this said, I understand I may have to grant this access to other files and possibly even directories (if all the files in that directory require read/write permissions.)
PS. id242, I checked and both my old working server and the new one has the SYSTEM access you had described on the "includes" folder. The difference was the special user account that Plesk creates to read/write/execute web/domain files described above.
Last edited by luket; 05/03/2016 12:57 AM.