When you request this file via dispatcher, you'll see a request to the AEM publish as well (the request created by the dispatcher); what's the response code of AEM? Is it also 204 (it should)? In case it is, can you go to that publish instance, search this request in the "recent request" view and post the details here? I am especially interested in the servlet handling this request.
Because I am not aware that the ClientLibProxyServlet is able to return the statuscode 204; this statuscode is something I would rather expect in a WebDAV scenario.