Combobox value is not assigned to the model class (Hibernate Validator)

I am trying to load values to a select (or asbobox) dynamically from server side with Spring Boot. I am also validating this form with the help of Hibernate Validator.

I'm having a very annoying problem that I can not explain why it's happening. In my form I have two combobox (or select), the first one is setting the values, while the second one (the dynamic one) is not doing it. Try to even try to put everything manually, but the annoying problem persists.

   <form:form id="form" modelAttribute="nuevoEvento" method="POST">
    <div class="containerLeft">
        <table>
                <tbody>
                    <tr>
                        <td><Label>Nombre del evento</Label></td>
                        <td><form:input type="text" path="titulo" /></td>
                    </tr>
                    <tr><td colspan=2 style="text-align:center;color:#84FCFF;"><form:errors path="titulo"/></td></tr>
                    <tr>
                        <td><Label>Tipo de Evento</Label></td>
                        <td><form:select class="combobox" path="tipoEvento">
                                <form:option value="N/A">Seleccione el tipo</form:option>
                                <form:option value="Importante">Importante</form:option>
                                <form:option value="Informativo">Informativo</form:option>
                                <form:option value="Privado">Privado</form:option>
                            </form:select></td>
                    </tr>
                    <tr><td colspan=2 style="text-align:center;color:#84FCFF;"><form:errors path="tipoEvento"/></td></tr>
                    <tr>
                        <td><Label>Salon del evento</Label></td>
                        <td>
                        <form:select class="combobox" path="salonId">
                            <form:options items="${listaSalones}"></form:options>
                        </form:select>
                        </td>
                    </tr>
                    <tr><td colspan=2 style="text-align:center;color:#84FCFF;"><form:errors path="salonId"/></td></tr>
                    <tr>
                        <td><Label>Dia del evento</Label></td>
                        <td><form:input type="text" path="fechaEvento" id="txtfecha" readonly="readonly"/></td>
                    </tr>
                    <tr><td colspan=2 style="text-align:center;color:#84FCFF;"><form:errors path="fechaEvento"/></td></tr>
                </tbody>
            </table>
            <input type="hidden" name="usuario" value="${principal.name}" />

The model class of this form is the following:

@TimeRangeConstraints.List({
@TimeRangeConstraints(
    fechaEvento="fechaEvento",
    horaInicio="horaInicio",
    horaCulminacion="horaCulminacion"
)
})
public class NuevoEvento {
@NotEmpty(message="Como se llamara el evento?")
@Size(max=40, message="Titulo invalido")
private String titulo;
@StringComboBoxConstraints
private String tipoEvento;
private String salonId;
private String url;
@NotEmpty(message="Seleccione la fecha del evento")
private String fechaEvento;
@NotEmpty(message="A que hora inicia el evento?")
private String horaInicio;
@NotEmpty(message="A que hora termina el evento?")
private String horaCulminacion;

public NuevoEvento() {}

public String getTitulo() {
    return titulo;
}

public void setTitulo(String titulo) {
    this.titulo = titulo;
}

public String getTipoEvento() {
    return tipoEvento;
}

public void setTipoEvento(String tipoEvento) {
    this.tipoEvento = tipoEvento;
}

public String getUrl() {
    return url;
}

public void setUrl(String url) {
    this.url = url;
}

public String getFechaEvento() {
    return fechaEvento;
}

public void setFechaEvento(String fechaEvento) {
    this.fechaEvento = fechaEvento;
}

public String getHoraInicio() {
    return horaInicio;
}

public void setHoraInicio(String horaInicio) {
    this.horaInicio = horaInicio;
}

public String getHoraCulminacion() {
    return horaCulminacion;
}

public void setHoraCulminacion(String horaCulminacion) {
    this.horaCulminacion = horaCulminacion;
}

public String getSalonId() {
    return salonId;
}

public void setSalon(String salon) {
    this.salonId = salon;
}

}

The attribute that is bothering is salonId .... now, how am I loading those elements? Here they have, from the controller:

@Controller
@RequestMapping("/usuarios")
public class UserController {

@Autowired
SalonRepository salon;
@Autowired
UsuarioSalonRepository userSalon;

@GetMapping("/home")
public String goHome(Model model, Principal principal) {
    System.out.println(principal.getName() + " HA INICIADO SESION!!!");
    User logueado = (User)((Authentication)principal).getPrincipal();
    String userInfo = WebUtils.toString(logueado);
    model.addAttribute("userInfo",userInfo);
    model.addAttribute("principal",principal);
    return "/basics/home";
}

@GetMapping("/crearEvento")
public String irCrear(Model model, Principal principal) {
    model.addAttribute("nuevoEvento",new NuevoEvento());
    model.addAttribute("principal",principal);
    model.addAttribute("listaSalones",getSalones());
    return "/Usuarios/crearEvento";
}

@PostMapping("/crearEvento")
public String crearEvento(@Valid NuevoEvento nuevoEvento, BindingResult result,
        Model model, Principal principal) {
    System.out.println("CONTROLSITO: " + nuevoEvento.getSalonId());
    System.out.println("Nombre: " + nuevoEvento.getTitulo());
    System.out.println("Tipo: " + nuevoEvento.getTipoEvento());
    System.out.println("Id Salon: " + nuevoEvento.getSalonId());
    System.out.println("Dia: " + nuevoEvento.getFechaEvento());
    System.out.println("Hora Inicio: " + nuevoEvento.getHoraInicio());
    System.out.println("Hora Fin: " + nuevoEvento.getHoraCulminacion());
    System.out.println("URL: " + nuevoEvento.getUrl());
    if(result.hasErrors()) {
        model.addAttribute("principal",principal);
        return "/Usuarios/crearEvento";
    }else
        return "index";
}

@RequestMapping("/misEventos")
public String misEventos(Model model, Principal principal) {
    return "/Usuarios/misEventos";
}

@RequestMapping("/editarEvento")
public String irEditar(Model model, Principal principal) {
    return "/Usuarios/editarEvento";
}

@RequestMapping("/borrarEvento")
public String irBorrar(Model model, Principal principal) {
    return "/Usuarios/borrarEvento";
}

public Map<Integer,String> getSalones(){
    Map<Integer,String> salones = new LinkedHashMap<Integer,String>();
    Iterable<Salon> ls = salon.findAll();
    salones.put(0, "Seleccione un salon");
    int i = 1;
    for(Salon salon: ls) {
        salones.put(i, salon.getNombre());
        i++;
    }
    return salones;
}

}

If I give it submit, you can visualize the data that you enter, but not that of the "salonId" select ....

It must be a silly mistake, but believe me I can not see where this fails