It's weird because I use directly a simple URL "intent".
By default I think is chrome that be used for FTP URL
Therefore normally you can open a FTP URL.
Can you try with a record (not a task) -> URL/URI (FTP).
Also if you have some special characters in the user name or password, it's could be the problem.
record does trigger nfc task, but a system nfc app. see screenshot.
there's a space in the path, but it's replaced with the good characters and open fine in chrome.
i see you're french: did you had the opportunity to test ftp on a freebox? on which ftp server did you test?