netscape wont show form result

I. am testing a feedback form which consists of a single file with the standard '<form method="POST" action=<?php echo "$PHP_SELF" ?>>' It has problems when using ns4.7. If the form is filled in correctly it works and sends email, but it checks for required fields and correct email address and if there is a mistake ns shows a blank page with the following source: ---------- <TITLE>Missing Post reply data</TITLE> <H1>Data Missing</H1> This document resulted from a POST operation and has expired from the cache. If you wish you can repost the form data to recreate the document by pressing the <b>reload</b> button. ---------- The whole thing works fine in ie5.5 with correct output of error or success messages and sending of email. Same result on localhost (win98se/apache1.3.2/php4.0.6) and public (unix/zeus/php4.1.2) What am i missing? Have i got some setting in ns that is preventing this from working?