I had the same problem, I had an old hard drive full of junk and duplicates and the Duplicate Finder didn't remove any of those.
The solution, for me at least, was to change the folders owner to "Users".
Right click on the folder, Properties->Security tab->Advanced, at the Owner: click Change, enter Users, hit OK and Apply. Close the Properties dialog fully, right click again at the folder, Properties->Security tab->Add the "Users" group to the security tab, and check the Full control. Hit Apply and restart CCleaner.
