Filezilla Explicit FTP over TLS not working with Airport Extreme
Looks like Airport Extreme acts as an ftp proxy, see https://discussions.apple.com/thread/2047569
And to make use of this proxy for FTPS connections it has either to do SSL man-in-the-middle (which causes certificate check to fail) or to reject the SSL upgrade (e.g. AUTH TLS command). Another way would be to do only the login with TLS and switch then back to plain text (CCC command), then the proxy could maybe listen to PORT commands.

Further discussions see https://discussions.apple.com/message/21689470