When using the XML web part in SharePoint, I received the error:

The XSL specified by the XSL property or XSL Link property is not valid, or the XSL file cannot be accessed. Make sure you have specified a valid XSL style sheet for this Web Part, or if the XSL file is linked that it can be accessed by the server. For assistance, contact your site administrator. More about making XSL files accessible.

I received this error after using SharePoint Designer to update the default.aspx page by adding a dataview. The obvious is that there is invalid informtion in my XSL, but it was working fine before I made the updates to the page.

After further examination, it appears that the XSL editor has extra spaces at the top of the page. The <?xml version="1.0" encoding="utf-8"?> should be the very first item in the page, first line first column. It wasn't It was on line two and column 20 or so. So I deleted the extra line and spaces and it worked again.

For some reason SharePoint Designer thought it necessary to mess with my already existing and functioning code. 

Hope this helps!