This occurs when PIX sees a FTP connection being initated back to the originating host. If you want server - server file transfer , I suggest you to use FXP. For this to happen make sure the firewall allows TCP/ UDP 286 for this to happen. But check how it works for security concerns.
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...