Define a member in the class concerned:
private static final long serialVersionUID = 1113799434508676095L;
This solves the problem described in the message, by forcing the serialVersionUID of the local class to match what was put in the stream when you serialized the old version of the class, which would have been computed automatically over various class attributes if no serialVersionUUD field was present at that time.
It is possible that you altered the class in an incompatible way, which will then produce a different exception. See the Object Serialization Specification chapter on Versioning for details. However merely adding or removing a field doesn't have that effect.
