Problem resolved via Stuart suggestion.
Update UI from outside threads it'll be dangerous...you could be some unexpected effects. Then each time that i add elements on my ObservableCollection i have called this update on main thread in that way:
private void AddElementsOnMainThread(AdvertObjectView item){
MvxMainThreadDispatcher.Instance.RequestMainThreadAction((Action)delegate(){
ElementList.Add(item);
});
}
in that way i have no more that warning and my UI thread it's protected from unexpected surprise!