Hi,
I suppose this has something to do with the Itemid which is lost.
At first you say in your post that it happens when you click on "proceed to checkout" but on the footnote you say that it's when you add a product to the cart, so I'm not sure which one it is but I'll go with the first one
Could you try to change the line
echo $this->cart->displayButton(JText::_('PROCEED_TO_CHECKOUT'),'checkout',$this->params,hikashop::completeLink('checkout'),'');
by
global $Itemid; echo $this->cart->displayButton(JText::_('PROCEED_TO_CHECKOUT'),'checkout',$this->params,hikashop::completeLink('checkout&Itemid='.(int)@$Itemid),'');
in the file components/com_hikashop/views/product/tmpl/cart.php near line 125 ?
That should keep the itemid for the next page when you click on "proceed to checkout" and avoid the problem. If that fixes the problem, we will include it for next release.