Getting null response when trying to call makeEwsRequestAsync() - soap

Question

I have built an Outlook Web Addin using Office-JS API. The addin has been tested from various endpoints and I know it works. However on some other machines I received a null response from the Exchange2013 server when trying to call mailbox.makeEwsRequestAsync. The operation is of type GetItem
Here is my XML SOAP:
<soap:Envelope
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages"
xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
<soap:Header>
<RequestServerVersion Version="Exchange2013"
xmlns="http://schemas.microsoft.com/exchange/services/2006/types" soap:mustUnderstand="0" />
</soap:Header>
<soap:Body>
<m:GetItem>
<m:ItemShape>
<t:BaseShape>IdOnly</t:BaseShape>
<t:IncludeMimeContent>true</t:IncludeMimeContent>
<t:AdditionalProperties>
<t:FieldURI FieldURI="item:InternetMessageHeaders" />
</t:AdditionalProperties>
</m:ItemShape>
<m:ItemIds>
<t:ItemId Id="AAMkAGFhOWQ3MWI0LWI3OWMtNDUyNy1iN2VhLTk5ZTg3N2UwMzc3OABGAAAAAACRYBMGwHc0T5HJQ/j+HHxaBwC8uj9oQcTXT4ysVvJ0+bvCAAAAU7w5AADOMEgE+yLSTL4F+7TbwddyAAMnU1GOAAA="/>
</m:ItemIds>
</m:GetItem>
</soap:Body>
</soap:Envelope>
Why am I getting null reponse from server ?
UPDATE
This is the last response I got:
{"Body":null,"ErrorMessage":"The remote server returned an error: (404) Not Found.","StatusCode":0,"StatusDescription":null,"WasProxySuccessful":false}