Thank you @AzDude & @Basj for your swift responses, it is much appreciated.
@Basj : The answers to your questions are as follows:
1. OS Build: 19042.746
2. Curtains version: 1.15. I am using Object Desktop to install and manage the applications (including Curtains). I have done the following: All Stardock applications have been uninstalled; Object Desktop uninstalled, re-downloaded and reinstalled. Finally all Stardock applications re-installed - Curtains is the only one that failed.
3. I am installing Curtains as part of the Object Desktop launcher application; no settings or directories have been changed.