Hi,
Thank you for the URL. I had tried another product and didn't had the problem there but I do see it on this link.
And I even got the error message when looking at the AJAX request for the add to cart:
i.imgur.com/D5rsX2r.png
Change the line:
$oldfiles = explode('|',$oldvalue);
to:
if(is_string($oldvalue))
$oldfiles = explode('|',$oldvalue);
else
$oldfiles = $oldvalue;
in the file administrator/components/com_hikashop/classes/field.php and it will fix the problem.
We'll add the patch on our end too.