Just a stab in the dark here but does the client PC have a desktop level firewall? If so, remove it if possible and try to connect. Ran into a similar problem months ago but not sure if it was exaclty the same message. Just trying to help if I can.
I did find out what was the cause. It was on the client side using NAT with a single external address. Each remote host will need its own IP address because the concentrator can only establish one tunnel per IP address.
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...