Currently Being ModeratedSep 11, 2013 3:06 AM (in response to Jo Daniell)
Did this happen all of a sudden or after migrating Aperture to a new machine? The serial number cannot be migrated. If you moved Aperture with Migration Assistant, use the installer media to reinstall it and upgrade immediately to the current version using Software update.
And yes, when you upgraded Aperture from earlier versions and bought upgrade serial numbers and not full version serial numbers, you will need to enter all previous version serial numbers. Do you still have them? If not, try this Pro Application Replacement Serial Numbers
when the support site is up again.
If you did not migrate Aperture recently. look at my post here, how to fix the ProAppSystemID file: