Thank you for your quick reply! Your assumption was right on the money, and it pointed to a SUE (Stupid User Error). I had mistakenly understood "Custom Files and Folders" to mean customizations done to files and folders in the user interface, such as custom icons on folders, etc.
Once I selected the Custom Files and Folders check box, CCleaner works as expected.
Sorry for wasting everyone's time. I am suitably shamed by my error.