GWT - The response could not be deserialized - gwt

Question

i am in this situation:
#RemoteServiceRelativePath("create_event")
public interface CreateEventService extends RemoteService {
String[] createeventServer(LinkedList<LinkedList<String>> input) throws IllegalArgumentException;
}
public interface CreateEventServiceAsync {
void createeventServer(LinkedList<LinkedList<String>> input, AsyncCallback<String[]> callback)
throws IllegalArgumentException;
}
public class CreateEventServiceImpl extends RemoteServiceServlet implements CreateEventService {
public String[] createeventServer(LinkedList<LinkedList<String>> input) throws IllegalArgumentException {
String[] arr = new String[2];
...
return arr;
}
Why does this cause error "The response could not be deserialized"?
p.s. I have tried to execute the project with app engine and without it, but the problem is the same.

Answer 1

To problems with serializable objects, you can try this check list:
Verify that the class has a default constructor (without arguments)
Verify that the class implements Serializable or IsSerializable or
implements an Interface that extends Serializable or extends a class
that implement Serializable
Verify that the class is in a client.* package or …
Verify, if the class is not in client.* package, that is compiled in
your GWT xml module definition. By default is present. If your class
is in another package you have to add it to source. For example if
your class is under domain.* you should add it to xml as . Be aware
that the class cannot belong to server package! More details on GWT
page:
http://code.google.com/webtoolkit/doc/latest/DevGuideOrganizingProjects.html#DevGuideModuleXml
If you are including the class from another GWT project you have to
add the inherits to your xml module definition. For example if your
class Foo is in the package com.dummy.domain you have to add to the
module definition. More details here:
http://code.google.com/webtoolkit/doc/latest/DevGuideOrganizingProjects.html#DevGuideInheritingModules
If you are including the class from another GWT project released as
a jar verify that the jar contains also the source code because GWT
recompile also the Java source for the classes passed to the Client.
Font: http://isolasoftware.it/2011/03/22/gwt-serialization-policy-error/