If you want to submit Ajax.BeginForm created ajax form a simple this.form.submit() wont work. Well to be exact it will, but not as it was intended. In order to make it work like expected, we can do it like this:
 
Html.DropDownList("selPredavanja",ViewBag.Lectures as SelectList,new { onchange = "Sys.Mvc.AsyncForm.handleSubmit( this.form, new Sys.UI.DomEvent(event), { insertionMode: Sys.Mvc.InsertionMode.replace, updateTargetId: 'lectureData' });"})