93.5 times out of 100 it's because the packet being returned to the server is not xml complient.
In other words something in the browse has "allow xHtml" ticked on, but the contents of that field are not valid xHtml.
This is easy to detect though;
a) in firefox run firebug (
www.firebug.com)
b) click the next button
c) in the "Console" tab in firebug, you'll see the response.
d) click on the little + to expand the response.
e) at the very top of the response you'll see an "xml" tab appear.
Click on this tab, and see if it's reporting an xml error.
Cheers
Bruce