I have same problem with you.. was... :-)
here's the solution:
1. Go to Control Panel in member area, scroll down to bottom, then you'll find "Account Tools"
2. Ok, see the "Fix File Permission" there? click and do just what u must do
3. If the problem still exist, click "Fix File ownership" too and execute
4. try to change the file/folder permission with file manager again
hope this will help
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