To find out why the procedure does not work on your system:
1) add the admispconfig settings for sudo.
to become ispconfig user, so you get the same resiults as the controlpanel. Then change to a web directory:
and execute the command:
du -h --max-depth=1
and check if the output is correct or if you get any error messages. ISPConfig is just displaying the output of this cammend in the interface.