I recently came across a pretty weird error. I had a standard ASP.Net page with some HTML markup, some .net controls, etc. , the page was working just fine in IIS7, but in IIS6 I was getting this error in FireFox: XML Parsing Error: no element found, and in IE the page was just loading blank.
After spending hours pulling my hair out trying to figure out this problem, I noticed that ONLY in IIS6 when the page was loading and in the OnLoad() method of the page, this.Page.Visible was being set false …. ! This was causing no HTML to be rendered, not even the default form that was included on the page, and thus was creating the error that I was seeing. FireFox apparently tries to parse a blank page as XML is why I was seeing the error there.
Anyways if this saves anybody any headaches… try checking that your page is coming in as Visible.
If anyone has any idea why this might have been happening to me, I would be absolutely interested to hear! :)