same reply as above Jacek;
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something in your browse is not xHtml complaint.
if you have added anything to the xHTML tabs, or you've ticked on the "allow xHtml" anywhere, then you are responsible to make sure that code _is_ xHtml complaint.
One way to "see" the rely you're getting back, after clicking on the Next button, is to use Firefox, with Firebug (
www.getfirebug.com), then in the "console" tab you can expand the request, and at the top of that response is an "xml" tab. click on that to see where your html is wrong.