It seems the author didn't give too much insight into why this problem occurred, saying only it was a server config problem.
The root problem behind the situation is the fact that the server does not have the .xsl filetype listed in its mime.type file list, telling browsers how to interpret certain file types.
If you have root access to your server:
I did it the manual way by editing the mime.type file in the /etc/nginx folder.
In the line with:
text/xml xml rss;
Just change it to
text/xml xml xsl rss;
Then restart the nginx service (e.g. service nginx restart)
For other HTTP handlers, just look up how to add mime types.
After you make the change, make sure to clear your browser cache or else the error will still show up. This seems to affect firefox users only since other browsers have methods of interpreting the xsl file to compensate for not so proper standards.
with regards to shared hosts (e.g. bluehost), your admin panel normally should have an option to add mime-types... e.g. cPanel has it listed under the Advanced tab. However I believe most shared hosts have a much wider range of mime types than base packages offered in unmanaged default packages (e.g. fresh install of apache, nginx, etc).