I purchased Vuescan X64 3/22/2014 Standard Edition, however when I used it today, I was asked to purchase it. I scanned a picture and it had a watermark all over it.
All the registration issues should be handled by the developer of the application. He has the possibility to check the registration status of your license. It's possible that the license expired. I recommend contacting him using the email address found on the support page provided by them.