Microsoft ISA 2006 Error 995

We are trying to use a link on a website which redirects the session to another website using javascript, this in turn inputs data into a form.  However, the ISA server (which is Windows 2003 SP2) is failing to make the connection and is posting the following error:

Log type: Web Proxy (Forward)
Status: 995 The I/O operation has been aborted because of either a thread exit or an application request.  
Rule: Allow HTTP out from Internal
Source: Internal (<ip address>)
Destination: External (<ip address:443>)
Request: external-site.net:443
Filter information: Req ID: 0c48df84; Compression: client=No, server=No, compress rate=0% decompress rate=0%
Protocol: SSL-tunnel
User: anonymous

I have installed ISA 2006 Service Pack 1 and run windows update, none of which have resolved the issue.  If I use a laptop on our LAN and go through the proxy it fails, but if I move the laptop onto an ADSL connection the redirect works fine.

I have also tried adding an any to rule into the ISA firewall, which made no difference.

Has anyone come across similar issues previously?  Could the javascript be written in such a way that the ISA server cannot handle it correctly?