Run FlashFXP as an Administrator to ensure it has permission to write the license data to the Windows Registry. Why You Should Avoid "Cracked" Fixes
Since FlashFXP is no longer reliably supported, it is safer to switch to modern, actively maintained FTP clients: flashfxp 540 build 3956 license key fix
Ensure your Windows Firewall isn't blocking FlashFXP from accessing the internet. Run FlashFXP as an Administrator to ensure it