I am unable to get a Category Module to work when Search Engine Friendly URLs is enabled.  All other menus/modules on the site work correctly with SEF URLs.  To highlight this problem I created a BRAND NEW Joomla 2.5.6 site.  The only component installed on the site is HikaShop Business 1.5.8.  I created two categories with one product in each category.  I created a Category Content Module and placed it in the right side module position.
When I go to the front end of the site I see the Category module displayed.  When I click on either of the two categories I receive a 404 error.
When I disable Search Engine Friendly URLs everything works correctly.
I also created Products Listing Menu items.  These work properly and show the appropriate products in the category with or without SEF URLs enabled.
Caching is disabled on the site