javax.xml.bind.JAXBException: Class *** nor any of its super class is known to this context

JAX-RS implementations automatically support marshalling/unmarshalling of classes based on discoverable JAXB annotations, but because your payload is declared as Object, I think the created JAXBContext misses the Department class and when it’s time to marshall it it doesn’t know how.

A quick and dirty fix would be to add a XmlSeeAlso annotation to your response class:

@XmlRootElement
@XmlSeeAlso({Department.class})
public class Response implements Serializable {
  ....

or something a little more complicated would be “to enrich” the JAXB context for the Response class by using a ContextResolver:

import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.ext.ContextResolver;
import javax.ws.rs.ext.Provider;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;

@Provider
@Produces({ MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML })
public class ResponseResolver implements ContextResolver<JAXBContext> {
    private JAXBContext ctx;

    public ResponseResolver() {
        try {
            this.ctx = JAXBContext.newInstance(

                        Response.class, 
                        Department.class

                    );
        } catch (JAXBException ex) {
            throw new RuntimeException(ex);
        }
    }

    public JAXBContext getContext(Class<?> type) {
        return (type.equals(Response.class) ? ctx : null);
    }
}