SEO URL = http://www.materiallistpro.com/downloads/autocad-2015-2016.html  (from your config page)

ORIGINAL URL = http://www.materiallistpro.com/index.php?id=122&lang=en&option=com_content&view=article&Itemid=564 (from your config page and inspect with firebug)

404 page error URL = http://www.materiallistpro.com/downloads/index.php  (Note that "downloads" is included in this URL, but not the original.)

"downloads" is a joomla category and a menu name, so I am not sure which affects the SEO URL.
I am using sh404SEF.