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

I'm trying to load values ​​to a select (or likebobox) dynamically from the 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't explain why it's happening. In my form I have two comboboxes (or select), the first one is setting the values, while the second one (the dynamic one) is not. I even tried 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 are, 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 submit, you can view the data that you enter, but not that of that "salonId" select...

It must be a silly mistake, but believe me I can't see where this is wrong