I'll have a chat with our support team about this and find out who you talked to.
We do have some customers who've had a intermittent issues with accessing kerboodle! via ISA server.
In these situations there appears to be a problem when the ISA server attempts to authenticate the users against the domain controller and a 12209 error appears in the ISA server logs.
On the kerboodle! side of things for some reason this results in a POST request with a Content-Length of zero and hence an error (I'm not sure if anything other than the first packet of the POST arrives at our servers).
Some customers have worked around the issue by allowing anonymous access to kerboodle! but we would like to get to the bottom of this issue.
I don't think you're likely to be suffering from the same issue but does your Smoothwall authenticate the user against a domain controller before authorising a requests?
Andy