Wordpress post form goes to 404 page - wordpress

Question

I'm coding a plugin that requires forms but I'm having some troubles on post sends.
I had read that the fields names can be a problem for this... But I have 3 fields: cmbParkings, cmbTarifas and dpkFechaEntrada.
My plugin register one custom post type called parking-parkia and one taxonomy for this post type with Ciudades as name.
I don't see where the problem can be! Maybe the field values could be a problem?
Explaination of fields:
cmbParkings is a select filled with all my custom posts (value=id, text=title).
cmbTarifas is dependant from cmbParkings, it loads a metadata from the parking-parkia previously selected (by the moment, value and text = metadata value, containing any characters)
dpkFechaEntrada is a date input.
If I do print_r($_POST) on my 404 page, I see my 404 loaded with the correct values from my form in the $_POST variable.
EDIT: I had comment all fields in form and 404 page still appearing on submit.
Form without fields:
<form id="frmFormularioBusqueda" method="post" action="http://mutuaparkia.extrasoft.es/?p=2632">
<div div="divBotonReserva">
<button type="submit" id="btnEnvioParking">Reservar</button>
</div>
</form>
Form with fields:
<form id="frmFormularioBusqueda" method="post" action="http://mutuaparkia.extrasoft.es/?p=2632">
<div id="selectorParking">
<select id="cmbParkings" name="msolla-cmbParkings">
<option value="0">Elige Parking</option>
<option value="2632">Parking Goya</option>
<option value="2633">Parking Córdoba</option>
<option value="2631">Parking Montalbán</option>
</select>
</div>
<div id="divSelectorTarifa">
<select id="cmbTarifas" name="msolla-cmbTarifas">
<option value="0">Elige Tarifa</option>
</select>
</div>
<div id="divSelectorFecha">
<input id="dpkFechaEntrada" name="msolla-dpkFechaEntrada" type="date">
</div>
<div div="divBotonReserva">
<button type="submit" id="btnEnvioParking">Reservar</button>
</div>
Note: the action url exists and it is ok.
Js that change the action form when the first select is changed:
$( document ).ready(function() {
$("#cmbParkings").change(function(){
//http://mutuaparkia.extrasoft.es/?p=2632
if($.isNumeric($("#cmbParkings").val())){
//$("#frmFormularioBusqueda").attr('action', '/?p=' + $("#cmbParkings").val());
$("#frmFormularioBusqueda").attr('action', $("#txtPermalink" + $("#cmbParkings").val()).val());
}
else{
$("#frmFormularioBusqueda").removeAttr( "action" )
}
$("#cmbTarifas").html($("#cmbTarifas" + $("#cmbParkings").val() ).html());
});
});
Edit: inside if, there is a commented line that it was bad. The new line is working fine now.

Answer 1

Thanks to vard, he had seen that changing the action from from url/?p=id
to the custom post permalink, the code works.