Plesk 17更改/添加httpdocs以进行网站更新

Sometimes I 'll do a major update on a website by creating an extra directory in the root of the subscription like httpdocs2 (Standard is httpdocs). Then I upload the new website to this directory, add a tempaorary domain (just for testing) to this subscription with httpdocs2 as the documentroot.

So far so good. After testing I change the original domain itself to httpdocs2. After that the website is working well but when I am trying to write to a folder (this is thumbnail generating code) I get:

Can't write image data to path (/var/www/vhosts/maindomain.com/httpdocs2/public/_files/.thumbs/image.jpg) 

It works perfectly on the original httpdocs so it has something to do with permissions.

(_files and underlying folders have 777 permissions)