I just noticed the following:

There is NO error when the following URL is used
HTML Code:
http://MyDomain.com/sitemap.xml
But when using this URL, the error gets generated!
HTML Code:
http://www.MyDomain.com/sitemap.xml
HTML Code:
Error loading stylesheet: An unknown error has occurred (805303f4)
http://MyDomain.com/includes/templates/template_default/css/gss.xsl
What causes this?