You can fix this by resetting the application to factory default settings as follows:
- Close sign | pro PDF
- Open an Explorer window, then copy and paste the following line into the address bar: "%APPDATA%\Wacom"
- Delete the folder "sign | pro PDF," and restart the application
- You will then be asked to select or create a digital certificate to be used with the application.
Once these steps are completed, you should no longer experience this issue.