Unfortunately, all the requests related to serial number activation or registration issues need to be handled by the developer. The problem is that the Sibelius developer was acquired by AVID. AVID has a support page available, but the only way to get in touch with them is through phone. You can access the page for more information. Additionally, they charge for Support. Every support actions has plans. This is rather inconvenient, but unless you contact them, the problem with the serial number can't be fixed.