Валидатор пропустил, но при самостоятельном запуске - invalid stream header..

public static void main(String[] args) throws Exception {
    Solution sol = new Solution("D:/sol.txt");
    sol.writeObject("smth");


    ObjectOutputStream obj = new ObjectOutputStream(sol.stream);
    obj.writeObject(sol);
    obj.close();

    FileInputStream fis = new FileInputStream("D:/sol.txt");
    ObjectInputStream obje = new ObjectInputStream(fis);

    Solution loadedObj = (Solution) obje.readObject();

    loadedObj.writeObject("smth");

}