404 error for Global site search

http://ep.experiencedesign.hk/site-search/

it work the first time you enter something, like drawing, than the next time you hit search again, even using some other terms, it will direct you to a link with the previous search term, e.g. http://ep.experiencedesign.hk/site-search/drawing, and there is a 404 error in this page, so it don't work