Hi,
What's interesting is that the text at the top is cut in the middle by the DIVs from the template:
i.imgur.com/6McH8OS.png
You can see that the text "Tak for din ordre.Du kan nu få adgang til din ordre ." is at the beginning of the body tag, next you have HikaShop's hidden footer tag (with the version number), and then you have HTML comments and scripts from the template. Then, you have some DIVs of the template, and then you have the "her" link to access the order and finally you have CSS and JS files which are supposed to be in the head of the page, not the body.
However, this should not be possible since the text and the link come from the same translation key:
YOU_CAN_NOW_ACCESS_YOUR_ORDER_HERE="Du kan nu få adgang til din ordre <a href='%s'>her</a>."
So what's likely is that you have a plugin of the group "system" which process the pages and find something here which lead it to go haywire and messes the whole page.
So I think what you should do is to disable the plugins one by one, and each time place a free order and see if that helps. Once you identify the problem, you can contact the developer of the plugin so that he can check what's going on.
Now, what you can do as a quick fix, is to edit the file checkout / end.php via the menu Display>Views and change the code:
echo JText::_('THANK_YOU_FOR_PURCHASE');
if(!empty($this->url))
echo '<br/>'.JText::sprintf('YOU_CAN_NOW_ACCESS_YOUR_ORDER_HERE', $this->url);
to:
JFactory::getApplication()->redirect('XXX');
where XXX is the URL of a page where you want to redirect the customer after a free order.
For example, you could use:
JFactory::getApplication()->redirect($this->url);
to directly redirect to the order details page of the order.
That should hopefully circumvent the problem.