POST JSON fails with 415 Unsupported media type, Spring 3 mvc

I am trying to send a POST request to a servlet. Request is sent via jQuery in this way:

var productCategory = new Object();
productCategory.idProductCategory = 1;
productCategory.description = "Descrizione2";
newCategory(productCategory);

where newCategory is

function newCategory(productCategory)
{
  $.postJSON("ajax/newproductcategory", productCategory, function(
      idProductCategory)
  {
    console.debug("Inserted: " + idProductCategory);
  });
}

and postJSON is

$.postJSON = function(url, data, callback) {
    return jQuery.ajax({
    'type': 'POST',
    'url': url,
    'contentType': 'application/json',
    'data': JSON.stringify(data),
    'dataType': 'json',
    'success': callback
    });
};

With firebug I see that JSON is sent correctly:

{"idProductCategory":1,"description":"Descrizione2"}

But I get 415 Unsupported media type. Spring mvc controller has signature

    @RequestMapping(value = "/ajax/newproductcategory", method = RequestMethod.POST)
public @ResponseBody
Integer newProductCategory(HttpServletRequest request,
        @RequestBody ProductCategory productCategory)

Some days ago it worked, now it is not. I'll show more code if needed.