I'm assuming you're in AFNetworking 2.0, which I haven't actually used. But it looks like you should be able to modify the responseSerializer property in the AFHTTPRequestOperationManager to accept whatever content type your server is sending. AFHTTPResponseSerializer has an acceptableContentTypes field you can modify.
However, given the error you're seeing it seems like the server may be returning an "application/json" content-type but not actually delivering valid JSON content.