GWT RPC XsrfToken Serialization Exception

I am working with gwt / app engine and the problem occurred since upgrading to App Engine 1.6.4 / GWT 2.4.0. I am using Java for an application. I see serialization issues. I'm not sure if the update caused this problem - it broke my application and I had to recreate the Eclipse project.

I am using GWT XsrfToken

with GWT RPC

. It works fine in Eclipse in development mode, but when I deploy my application to the application, I get the following error in the application engine log:

javax.servlet.ServletContext log: Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.google.gwt.user.client.rpc.XsrfToken' was not assignable to 'com.google.gwt.user.client.rpc.IsSerializable' and did not have a custom field serializer.For security purposes, this type will not be serialized.: instance = com.google.gwt.user.client.rpc.XsrfToken@1e80c87
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serialize(ServerSerializationStreamWriter.java:619)
    at com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamWriter.writeObject(AbstractSerializationStreamWriter.java:126)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter$ValueWriter$8.write(ServerSerializationStreamWriter.java:153)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serializeValue(ServerSerializationStreamWriter.java:539)
    at com.google.gwt.user.server.rpc.RPC.encodeResponse(RPC.java:616)
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForSuccess(RPC.java:474)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:571)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:208)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:248)
    at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
...more 

      

        
        
        
      

    

Any help would be much appreciated.