Searching for a "license key fix" or "crack" for FlashFXP 5.4.0 (Build 3956) typically leads to unstable or malicious software. FlashFXP was a popular FTP client that is now effectively discontinued
Contact Support: If you're using a legitimate copy of FlashFXP, reaching out to their official support might provide the most straightforward solution. They can offer specific guidance or patch files if needed.
Security Warning: Any "fix" or "crack" found on third-party sites for this build often contains malware. Given that the software is no longer updated, using unauthorized "fixes" leaves your system and server credentials highly vulnerable to theft. Critical Review & Verdict FlashFXP v5.0 version history - Windows FTP Client
Developer Status: The developer (Charles DeWeese/Bigstar) was sentenced to federal prison in 2017, effectively ending all future updates and official support.