Have you received any error messages once the VPN traffic has stopped. Check if the router encrypts normally when hardware encryption is disabled with the "No crypto engine accellerator" command.
The ah_seq_fail or esp_seq_fail error counts increment in the output of the show crypto engine accelerator statistic, if there are Packet Encryption/Decryption errors. This happens when the VPN card is placed under stress using fast switching with a mixture of fragmented and unfragmented packets. If it is a fragmentation issue, then refer to the following document which gives you the details of the pre-fragmentation feature.
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...