That's wierd. Besides a few sites, the only other thing I have on my site is AutoresponsePlus. Huh. If anyone has any ideas please post. I don't even know what mrtg is... :p
Hia,
MRTG is The Multi Router Traffic Grapher. It is a very useful stuff. There should be a page in your server that will give you a graph of the server stats. This is the mrtg.
The mrtg needs a work directory to which it produces the graphical pages. This directory is missing in your case and hence the error.
Solution: Just make the directory /var/www/html/mrtg in your server. Everything will be back to normal