This looks like a serialization 'problem', or better: the system prevents you from having one.
As you are using a different version of the server, you use different versions of classes. Therefore the serialVersionUID has changed. This is detected by the JVM when loading the object. It gets an object with an old serialVersionUID and throws the exception.
Guess you'll have to delete the old serialized objects.
Thanks for your help, the problem seemed to go away by itself in the end, so it could well be what you said, looks like Ill never know.