Esempio

public static class SerializableObject implements Serializable{

synchronized private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
}
}

Soluzione
Dichiarare readObject() senza il modificatore synchronized

public static class SerializableObject implements Serializable{

private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
}
}