The problem looks like you have Admin Tools installed on your Joomla! website with a customized version of the .htaccess file.
Indeed the special version of the .htaccess file generated by Admin Tools blocks certain folders and files thus making impossible to retrieve XML stylesheet files needed for the XML formatting.
To fix this issue, you can search for the following line in the .htaccess file:
RewriteRule ^(components|modules|templates|images|plugins|media|libraries|media/jui/fonts)/ - [F]
and remove the 'components' folder to have the following result:
RewriteRule ^(modules|templates|images|plugins|media|libraries|media/jui/fonts)/ - [F]