The first problem with Sibelius might be that you are booting from an old alias which is pointing to a previous version. If you have a shortcut try deleting it and booting from the original Sibelius icon in the folder.
Legacy Forum Post
about 17 years ago
I did a clinic the other day where a user had a similar problem. As it turns out, he had somehow installed the program three times, and it was living in a variety of locations on his hard drive. One version had been registered, the others weren't. So depending on which version of the program launched, he'd sometimes get the message that he wasn't registered, etc.
It might be worth doing a "Find" for Sibelius on your hard drive just to see if there are multiple versions installed somehow. If so, select each file and do a "Get Info" on it to verify which is the most current version.