This is usually caused by other software on that machine, usually firewall software. If you have any personal firewall apps, remove that, then reinstall the VPN client and see if that works. Make sure ICS is turned off also.
Basically something is interfering with the VPN client, so you need to get rid of that.
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...