Office 2013 Outlook app: The requested web method is unavailable to this caller or application.

Currently I'm busy developing an Office 2013 outlook app. I am trying to get the attachment from a mailbox item via the mailbox.makeEwsRequestAsync method. The response I receive has the following error: The requested web method is unavailable to this caller or application.

I used the following envelope.

<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">  <soap:Header>     <t:RequestServerVersion Version="Exchange2013"/>  </soap:Header>  <soap:Body><GetAttachment xmlns="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types"><AttachmentShape><IncludeMimeContent/><BodyType/><FilterHtmlContent/><AdditionalProperties/></AttachmentShape><AttachmentIds><t:AttachmentId Id="AQMkADhkMDRiYzlmLThjN2QtNGM0My05ODMwLTIyAGNmNDI0ZDMwODAARgAAAxnFAoqua3ZGnLV5PgcmjYQHAPCtHV4zyh1HqhQzDmo8SGUAAAMOAAAA8K0dXjPKHUeqFDMOajxIZQAAAysAAAABEgAQAHv5ksiBZM5BsVkyxDDLeIU="/></AttachmentIds></GetAttachment>  </soap:Body></soap:Envelope>

Cheers,
Mathijs