Can you post any of the config from the PIX and router. Is there any NAT involved.
The fact that you say the first packet works tells me that process switched packets are getting encrypted correctly, but fast switched packets do not get processed correctly. You can test this by disabling fast switching. Another possible solution would be to use CEF.
Do a show ip int brief to determine your switching method.
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...