Navigate to the QuarkXPress 6.5 folder, create a new folder, and name it Preferences. If the Preferences folder already exists in the QuarkXPress folder, delete the contents of the Preferences folder and relaunch the application.
This is an 'erroneous error' related to a permissions restriction on the Preferences folder when it is located in the default location. Creating the Preference folder in the QuarkXPress folder will resolve the problem. QuarkXPress 6.5 in this case was picking up a message from the error stack for QuarkXPress 4.11, which was still installed on the machine.