I believe that I have been bitten by this issue before myself. I believe that if you look carefully when you attempt to login and the TACACS servers are not available, that the error message that you get is authorization failure where we would generally expect to see authentication failure.
I believe that the issue is in your configuration of aaa authorization. You currently have this configured:
aaa authorization exec default group tacacs+ local
I would suggest that you change it to this:
aaa authorization exec default group tacacs+ if-authenticated
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...