Custom taxnomies not working on WP 3.5.1

Hi guys,
i have installed WP version 3.5.1.
I have a Custom post type with a custom taxonomy field that has 2 values.
When i try to filter records with tax_query in the wp_query, i don’t get any results or i get results but filtered on the other condition present in the wp_query.

How can i resolve the problem?
i use this code:

$args = array(
							'post_type' => 'eventi',
							'post_status' => 'publish',
							'meta_query' => array(
								array(
									'key' => 'data_evento',
									'value' => $data_odierna,
									'compare' => '>=',
									'type' => 'DATE'
								)),
							'tax_query' => array(
									array(
									'taxonomy' => 'categorie_eventi',
									'terms' => array('formazione'),
									'field' => 'slug'

									)
							),
							'meta_key' => 'data_evento',
							'orderby' => 'meta_value_num',
							'order' => 'DESC'

						);

thanks in adavance

Giulio