An issue with the Classifieds plugin shortcode

I've set up the Classifieds plugin on my site http://www.inmillerton.com. I used the shortcode to list the categories on this page: http://www.inmillerton.com/bulletin-board/home/
It was working before but now is not - clicking on a category returns a page not found. The same result is if I click on 'view' from the Edit Categories admin page. Can someone please look into this and let me know what the source of the problem is? Thanks.

M