The Windows 8.1 update seems to trigger the system that CheckDesigner uses to validate installations. Some customers have indicated that if they try to enter their original serial number at this prompt, it will not accept the serial number.
To resolve this issue, you will simply need to uninstall and then reinstall CheckDesigner. Doing this, you will be able to successfully enter the serial number when prompted this way. We have confirmed this as a working solution with all customers that have been affected by this issue.