It may be to do with a NAT overload ie. where you translate multiple private IP addresses to one public IP address.
With this setup the firewall doesn't just translate the IP address but also has to modify the port number. Because there is a finite number of ports ie. 1 - 65535 it may be that the firewall has run out of available ports ie. there are too many private addresses that need translating for the available ports.
Note that not all of the 1 - 65535 port numbers are available to the firewall when doing NAT overload.
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...