Thanks, Jerre, for your clear answer to this problem...

My question must look ridiculous to you (I know nothing about php editing - I just follow instructions), but could you please explain me where to add the last line.

My editlib.php looks like this:

          $returnurl = $pageurl->out(false, array('category'=>"$tocategoryid,$contextid"));
            if (!$checkforfiles){
                if (!question_move_questions_to_category(implode(',', $questionids), $tocategory->id)) {
                    print_error('errormovingquestions', 'question', $returnurl, $questionids);
                }
                redirect($returnurl);
            } else {
                $movecontexturl  = new moodle_url($CFG->wwwroot.'/question/contextmoveq.php',
                                                array('returnurl' => $returnurl,
                                                        'ids'=>$questionidlist,
                                                        'tocatid'=> $tocategoryid));
                if ($cm){
                    $movecontexturl->param('cmid', $cm->id);
                } else {
                    $movecontexturl->param('courseid', $COURSE->id);
                }
                redirect($movecontexturl->out());
            }
        }
    }

    if (optional_param('deleteselected', false, PARAM_BOOL)) { // delete selected questions from the category
        if (($confirm = optional_param('confirm', '', PARAM_ALPHANUM)) and confirm_sesskey()) { // teacher has already confirmed the action
            $deleteselected = required_param('deleteselected');

Where exactly should I paste it?