You are working trough ftp client and you can change the file permission from specific file due to a user file creation owner. its usually to see that the file owner/group is different.
Solution:
Access to Plex server trough SSH client and change the file permission using chmod command.
how to access httpdocs when you are logged in the server trough SSH ?
on the first ls -la you will see something like that:
- master
- parallels
- parallels_installer
- plex-installer
to open httpdocs folder you have to open: /var/www/vhosts from the home folder
source:
http://support.hostgator.com/articles/plesk/plesk-administration/using-ssh-in-plesk