Can you please email me at
support@flashfxp.com with the name & email address on the registration key. I was unable to find your FlashFXP ID in our database.
Please make sure you're using an authentic release of FlashFXP, unofficial or modified versions will generate invalid ID's and may also explain the problem you are experiencing.
Uninstalling wont help, it doesn't remove any of the settings/data files. Installing over a previous version shouldn't cause any problems, however problems to arise. You could try deleting your FlashFXP.ini and accept loosing your settings.