I have just spent a while wrestling with this problem, so in case anyone else comes across it.

Symptoms

• You are making an AJAX query to your server – in my case using Yahoo’s excellent YUI, connection manager.
• On IE6, using the Microsoft.XMLHTTP object, you try and read the responseText parameter (or the YUI does for you).
• IE says msxml3.dll: System error: -1072896658 (nice error eh?)
• Everything works fine in Firefox

Solution

Make sure the response from your server has the correct mime type: ‘text/plain’ if it really is plain text. In my case I was sending a response of ‘text/html’ and sending plain text.

I wish you never experience this problem. If you do, hopefully this has helped you.