Debug info: 

Error code: setting_invalid_ui_label 

$a contents:

Stack trace:

line 143 of \backup\util\ui\backup_ui_setting.class.php: base_setting_ui_exception thrown

line 156 of \backup\moodle2\backup_section_task.class.php: call to base_setting_ui->set_label()

line 132 of \backup\util\plan\base_task.class.php: call to backup_section_task->define_settings()

line 60 of \backup\util\plan\base_plan.class.php: call to base_task->set_plan()

line 159 of \backup\moodle2\backup_plan_builder.class.php: call to base_plan->add_task()

line 186 of \backup\moodle2\backup_plan_builder.class.php: call to backup_plan_builder::build_section_plan()

line 99 of \backup\moodle2\backup_plan_builder.class.php: call to backup_plan_builder::build_course_plan()

line 163 of \backup\util\factories\backup_factory.class.php: call to backup_plan_builder::build_plan()

line 62 of \backup\util\plan\backup_plan.class.php: call to backup_factory::build_plan()

line 357 of \backup\controller\backup_controller.class.php: call to backup_plan->build()

line 126 of \backup\controller\backup_controller.class.php: call to backup_controller->load_plan()

line 84 of \backup\backup.php: call to backup_controller->__construct()

$section_included->get_ui()->set_label(get_section_name($course, $section));

    public function set_label($label) {

        if ((string)$label === '' || $label !== clean_param($label, PARAM_TEXT)) {

            throw new base_setting_ui_exception('setting_invalid_ui_label');

        }

        $this->label = $label;

    }