Based on the debugs, it looks like the IKE packets are being blocked somewhere along the path between the Hub and Spoke. Make sure that the IKE and IPSEC Ports/Protocols are not blocked anywhere between the ASA5505 and headend side.
One document points to the key being invalid, another indicates the crypto ACLs aren't properly setup. Try re-entering the key on the spoke to verify it matches with the hub. Also double-check the crypto and nonat ACLs on both sides to verify they look proper.
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...