I'm trying to access the Royal Mail parcel tracker at
www.royalmail.com on a Windows 98 client using integrated
authentication to an ISA server running Websense.

If the user account I've logged in to AD with is a member
in a certain security group ("Web managers"), I can
access the tracker submission page a couple of times, but
after the 2nd or 3rd parcel read attempt I get a 404
error in response. I can, however, reload the tracker
page to submit the parcel number.

If the account is a member of a standard user group
instead of "Web managers", the number of times I can
submit a parcel before getting a 404, is completely
random, however more often than not it fails on the first
attempt.

This pattern can be replicated on any Win98 workstation
with a user logged into AD and using IE6.

I've exhaustively checked both AD and Websense. In both
cases, the Royal Mail website is basically set for open
access to all users - ISA doesn't cache it or block it,
and Websense has it defined as a custom site which is
allowed for all users.

The only group policy in existence is the domain group
policy. No administrative templates related to IE are
applied.

I've tried fresh install of IE6SP1 with latest security
updates, both with and without the Java VM. No joy.

Here's the funny thing: I installed Mozilla Firefox,
temporarily enabled basic authentication so it'd work,
and this works fine for all users.

So: What's likely to be the problem?

The authentication method?
The browser?
Or the fact that the workstations are set to use one
profile for all users who log into AD?

Or is there something else I've missed?