FlashFXP v3.x requires a new registration key, Previous users can obtain an updated registration key for free.
If you still have the original email address that you used to purchase FlashFXP you can enter it on the following page
https://secure.flashfxp.com/keyupgrade.php and receive your updated key instantly.
Otherwise you're going to need to contact
support@inicom.net and provide them with your old registration key and information.
BTW the installer clearly states that if you're currently using FlashFXP 2.x you'll need to upgrade your key and re-enter it.
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