Difficult to say why it's failing. You seem to get past XAuth, then around message 53 things go bad. Probably need to see the log from the concentrator at the same time, since the client is complaining about a packet it received from the conc.
Under Config - System - Events - classes add the following classes, all with Severity to Log of 1-13: AUTH, AUTHDBG, IKE, IKEDBG, IPSEC, IPSECDBG. Then go to Monitoring - Filterable Event Log, clear the log, try a connection then after it fails do a GetLog, look through that (probably nearer the end of the log) and see if anything's obvious.
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...