Summary: This issue occurs due to Accent present in the username.
1. Navigate to C:\Users\<home_user>\AppData\Local\Quark
2. Move the AnonymousData and QuarkXPress 2015 to the Desktop.
3. Now, Navigate to C:\Program Files\Quark\QuarkXPress 2015.
4. Create a new folder and name it 'Preferences'. (Note: This folder is Case-Sensitive).
5. In the location as Step 3, Open the file settings.xml with notepad.
6. Update the file by giving the preferences folder path in between <Win></Win> tags. Example- <Win>C:\Program Files\Quark\QuarkXPress 2015\Preferences</Win>
7. Save the file and launch QuarkXPress. If you get 'Access Denied' error while saving Settings.xml, please save it on desktop using Save As option.
8. Now, replace the old settings.xml file with the updated one.
9. Launch QuarkXPress 2015.