If you have issues with the serial number, then I recommend using the contact options found on the official website of the developer to get in touch with him. This will ensure you have a direct way to the developer to ask him about this error. It's possible that the serial number to be over-used and because of that you receive an error.
Contact information page: http://www.mysher.com/map_En.jsp
You have the possibility to contact it using the phone or the email address.