[Zope] Problem with Zope and Proxy server

I do most of my network access through a firewall with a Netscape Suitespot Proxy server and I now have a Zope 2.0 Beta 3 system set up outside the firewall. Mostly it works fine, but I find that attempting to submit a form with a POST method always fails with Error 400 (Not Found). This happens with any form I create but only if I use POST (changing the submit method to GET and it all works fine) and only if I access Zope via the proxy. The problem seems to be with the Netscape Proxy server interpreting the result from Zope as any side-effects on the Zope server do happen. Can anyone suggest what is happening here?