In version 1.0.2, the registration code system was slightly modified.
Each registration code is specific to a particular MAC address. When the MAC address used for the registration code is no longer available, the game requires re-registration. If the same thing happens again, but the original MAC address is now available, previous versions of the game asked for re-registration again (they only stored one registration code). From version 1.0.2 onwards, the game doesn't require re-registration the second time - it remembers the old registration code which is valid with the original MAC address.
This caused problems for some people who switched network adapters, or enabled and disabled their network connections. If this affected you, please contact me and I'll arrange extra registration codes to replace those used because of this problem.