The business object 0 in the namespace 1 is not an ATOM feed ("http://www.w3.org/2005/Atom/feed") or entry ("http://www.w3.org/2005/Atom/entry") business object and hence it cannot be serialized by the ATOM data handler.
Explanation
The ATOM data handler can serialize only an ATOM feed business object or an ATOM entry business object. The business object sent to it is neither one.
User response
Change the interface such that the parameters of the request/response type are either an ATOM feed or ATOM entry business object. The other option is not use the ATOM data handler but a propreitary data handler which can serialize the custom business object.