<Error> <Code>AccessDenied</Code> <Message>Access Denied</Message> <RequestId>E9F449356111EF63</RequestId> <HostId> rQvhUyG5YcEhWAybnmeDjIofcdmbmua7vkr/7v16J7EK3oQZPaQwBBlDFRZb54w7h3wz/F wfWA= </HostId> </Error>

Looks like XML.

Seriously, without any additional information we can only guess what you are doing.

To me it looks like API says Access Denied, but instead of doing this is JSON response, you receive XML response (for some reason).

Because the response you get here is similiar I get when I request data from API and something goes wrong with permissions (but like I already mentioned, I get hostId, requestId and hostId in JSON not in XML).

Pritaeas mentioned it already, would be great if you told us what you're doing.