Consolidating an One to Many relationship in Thymeleaf

I am trying to consolidate an One to Many relationship using Thymeleaf. Let's keep it simple. I do have a thiesis a thiesis has some questions.

@Entity
public class Thiesis {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;

    @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER, mappedBy="thiesis")
    private Set<Question> questions = new HashSet<>();

    @ManyToOne(cascade=CascadeType.ALL)
    private Course course;

    @Temporal(TemporalType.TIMESTAMP)
    private Date createdAt;
}

And the question class:

@Entity
public class Question {
  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
  private Long id;
  private String question;
  @ManyToOne(cascade=CascadeType.ALL)
  private Thiesis thiesis;
}

And here is the controller for saving the thiesis:

@PostMapping("/teacher/{userId}")
public String addThiesis(@PathVariable Long userId, Thiesis thiesis,String name)
{
    thiesisService.save(thiesis, name);
    System.out.println(thiesis.getId());
    return "redirect:/teacher/" + userId.toString();
}

Also the service for Thiesis:

public Thiesis save(Thiesis thiesis, String name)
{
    Course course =courseRepo.findByName(name);
    if(course!=null) {
        thiesis.setCourse(course);
        for(Question question : thiesis.getQuestions()) {
            question.setThiesis(thiesis);
            questionService.save(question);
        }
        Date date = new Date();
        thiesis.setCreatedAt(new Timestamp(date.getTime()));

        return thiesisRepo.save(thiesis);

    }
    else {
        System.out.println("Couldn't save the thiesis");
        return null;
    }

Thymeleaf:

<form action="" th:object="${thiesis}"method="post">
    <input type="hidden" th:name="${_csrf.parameterName}" th:value="${_csrf.token}" required/>
    <div class="form-group row" id="course_id">
        <label for="pyetja_1" class="col-12 col-sm-4 col-form-label">Pyetje:</label>
        <div class=" col-12 col-sm-8">  
            <input type="text" class="form-control" placeholder="Emri i Lendes..." th:field="*{course.name}" required/><br/>
        </div>                      
    </div>
</form>

But the result is not getting into database, and as a response I am getting a 403 error, when I perform the POST method. Probably the problem is at the way I am completing the object fields in thymeleaf. I am really stuck, and I have tried thousands of ways and I am not dealing with it. Any help is much appreciated.