21 is control channel for Explicit SSL, allowing the user's computer to send commands to the server. Now, this may be from client end in which case from inside ASAs allow outbound connection (default). You can give a try without port 21 and post back what worked for you.
Login to the FXOS chassis manager.
Direct your browser to https://hostname/, and log-in using the user-name and password.
Go to Help > About and check the current version:
Check the current version availa...
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...