Error sending array of inputs

As I said in a comment above, setting query_var => false for the custom taxonomy is not a solution because this will make term pages in frontend return a 404 error. Renounce to term pages in the frontend to make a simple form to work is far away to be a solution.

Finally I’ve solved the problem. Instead of set the name in the select element to ‘my_tax[]’, I’ve set it to something like ‘tax_input[‘my_tax’][]. Don’t ask me why WordPress needs this, but it fix the problem.

Just if someone needs an example.

My sample taxonomies:

$taxonomies = array(
            'type'               => array('label' => 'Types','slug' => 'type','hierarchical' => false,'multiple' => false),
            'city'               => array('label' => 'Cities','slug' => 'city','hierarchical' => true,'multiple' => false),
            'community_features' => array('label' => 'Community features','slug' => 'community_features','hierarchical' => false,'multiple' => true),
            'property_features'  => array('label' => 'Property features','slug' => 'property_features','hierarchical' => false,'multiple' => true)
    ); 

   foreach($taxonomies as $taxonomy => $properties){
        register_taxonomy(
                         $taxonomy,
                         'properties',
                         array(
                              'label'        => __( $properties['label'] ),
                              'hierarchical' => $properties['hierarchical'],
                              'query_var'    => true,
                              )
                          );

         register_taxonomy_for_object_type( $taxonomy, 'properties' );

}

Here a sample form:

           <form method="post" action="<?php echo get_post_type_archive_link('properties');?>">
             <?php
               foreach($taxonomies as $taxonomy) {
                   $args = array(
                                'show_option_all'    => __('Select '.$taxonomy['label'],'properties'),
                                'orderby'            => 'NAME', 
                                'order'              => 'ASC',
                                'hide_empty'         => 0, 
                                'echo'               => 0,
                                'selected'           => '',
                                'hierarchical'       => true, 
                                'name'               => 'tax_input['.$taxonomy['slug'].'][]',
                                'class'              => '',
                                'taxonomy'           => $taxonomy['slug'],
                             );

                 if($taxonomy['multiple']){
                   $args['show_option_all'] = '';
                 }
                 $select = wp_dropdown_categories( $args );

                 if($taxonomy['multiple']){
                   $select = preg_replace("#<select([^>]*)>#", "<select$1 multiple>", $select);
                 }

                 echo $select;
               }
             ?>
             <input type="hidden" name="action" id="action" value="search">
             <button type="submit"><?php _e('Search','properties');?></button>
           </form>

Added a custom query var:

add_filter('query_vars', 'properties_add_query_vars');
function properties_add_query_vars( $vars) {
   $vars[] = "action"; // name of the var as seen in the URL
   return $vars;
}

An my sample pre_get_posts:

add_action( 'pre_get_posts', 'properties_pre_get_post' );
function properties_pre_get_post($query){

  if(isset($query->query_vars['action']) && $query->query_vars['action'] == 'search'){

     if($query->is_main_query() && !is_admin() && $query->is_archive ) {

        $tax_query = array();

        $tax_input = $_POST['tax_input'];
        if(!empty($tax_input)){

           foreach($tax_input as $key => $value){
              if(array_key_exists($key, $taxonomies)){
                 if(!empty($value) && $value[0] != "0"){
                    $value = array_map('intval', $value);
                $tax_query[] = array(
                                         'taxonomy' => $key,
                                         'field'    => 'id',
                                         'terms'    => $value,
                                         'operator' => 'AND'
                                   );
                 }
              }
            }
         $query->set('tax_query',$tax_query);
       }
    }
  }
}