I had similar issue recently. Serfing the web I have found the solution which helped me:
Step 1: Always make a back up of the file you are working on - will save you a lot of time and nerves.
Step 2: Open the components/com_hikashop/router.php file with a text editor (I use Notepad++. Others might screw your files up due to the wrong encoding).
Around line 164 find
if(!in_array($name,array('option','Itemid','start'))){
and replase it with
if(!in_array($name,array('option','Itemid','start','format'))){
This did work for me.