How can I stop the dreaded 14120 error?
This FAQ is by Dr. Thomas W. Shinder:
The most common reason for the dreaded 14120 error is that you’re looping backup through the external interface of the ISA Server to access an internal network server that you published via ISA Server.
You can’t do that! Another reason could be that you have not created a split DNS infrastructure.
One way to get around creating a split DNS infrastructure is to create a HOSTS file on the ISA Server that contains the FQDN contained in the request host header.
The entry in the HOSTS file would contain that same FQDN, but it would map to an internal network server.
That way, http://www.mypublishedserver.com would resolve to an internal network IP address, instead of the public address on the external interface of the ISA Server