The problem is mainly caused by the denial of reply back traffic comming from the your FTP server. so there are two sollutions: firest in the DMZ access list allow traffic from FTP server to any on FTP ports OR, configure traffic inspection, you can use the default ASA inspection:
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...