The Failed/Retry logic is broken

[bigstar]

The action is based on the reply number, the standard defined meaning of an 550 is:

550 Requested action not taken.
File unavailable (e.g., file not found, no access).

While the ftp server you describe uses 550 for something non-standard, it must be treated as the standard and expected result.

In my opinion perhaps the ftp server should reply with

450 Requested file action not taken.
File unavailable (e.g., file busy).

If the error reply was 450 I believe FlashFXP would then retry the file.