There have been reports of serial number activation attempts failing after upgrading to the Mavericks OS. In some cases the problem was preceded by the serial number first becoming deactivated and all attempts to activate again fail.
In all cases Qube support has clarified that the license had been activated on the eSellerate license server – however, this activation does not seem to "stick" in the OS System Preferences.
This issue seems to be the result of some system-level change after an OS update to the Mavericks OS (10.9.x).
Currently the only solution is to reinstall the OS.