To be honest I have the same version and I have never had that problem so I really don't know. If I were you I would try upgrading/downgrading the PIX/ASA image and the ASDM and verify if the error persists.
Also, I would suggest you open a TAC case with Cisco. This is not a normal behaviour.
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...