FTP(S) and ISA Server, it’s allways a bi***
Not to mistake with SFTP btw, a kind of FTP over SSH.
When going through these posts:
Stil ended up with a 550 error while accessing FTPS Server with FTPS Client through ISA. (when server is getting AUTH SSL / TLS cmd.)
Ended up disabeling the ISA “FTP Access Filter” , not just for this rule only but for the whole ISA Server.
(expand: Server,Ad-ins, FTP Access Filter, Disable it. It took quite a while to complete, do restart the firewall service when it asks.).
NOTE !! (If you have existing FTP (not FTPS) Rules, you might have to adjust them too)