Hi Nicolas,
Let me try to assist Nico.
From what I understand, a view override is for a php file. So I don't understand why you're talking about a xml file.
You are partially right. Usually a template override is just a PHP file. However, with an Alternative Layout an XML file comes into play. A PHP file isn't even required then. The XML file makes it possible to modify behaviour of the view and adds an additional menu item type for creation of menu items. In our case we wanted some extra settings for the "Categories listing" view. So we created an Alternative Layout with an XML file named listing-custom.xml (.../templates/yootheme/html/com_hikashop/category/listing-custom.xml).
In my development environment this worked flawlessly. That site was a quite recent backup of Nico's installation. Apparently Nico changed something since that backup which now causes an access denied error. The link of the menu item causing the error looks like this: index.php?option=com_hikashop&view=category&layout=yootheme:listing-custom. As you can see, Nico uses YOOtheme and you can also recognize the name of the override's XML file. For reasons I am not aware of, the layout name is used by HikaShop as a task name and the task name is verified against an array with a limited set of names: list, show, cancel (.../administrator/components/com_hikashop/helpers/helper.php, line 2431). Obviously this fails, resulting in the access denied error.
Is this enough information for you to help us solve the problem?