Older Versions

Thanks for the link.
FYI: 4.8.1 still didn't fix my problem...

yes sure:

// register meta poxes for attend page in backend
add_filter( 'rwmb_meta_boxes', 'box_register_meta_boxes' );

function box_register_meta_boxes( $meta_boxes )
{
    $prefix = 'box_';

    // meta box
    $meta_boxes[] = array(
        'id'       => 'boxes',
        'title'    => 'Preview Boxes',
        'pages'    => array( 'page' ),
        'context'  => 'normal',
        'priority' => 'high',

        'fields' => array(
            
            array(
                'name'  => 'Featured Box',
                'desc'  => 'Full width featured boxes for content',
                'id'    => $prefix . 'group_big',
                'type'  => 'group',
                'clone' => true,
                
                // List of child fields
                'fields' => array(
                    array(
                        'name'    => 'Date',
                        'id'      => 'date',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Featured Text',
                        'id'      => 'text',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Image-URL (1.035x605)',
                        'id'      => 'url',
                        'type'    => 'url',
                    ),
                    array(
                        'name'    => 'Title',
                        'id'      => 'title',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Subtitle',
                        'id'      => 'subtitle',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Facebook Event URL',
                        'id'      => 'facebook',
                        'type'    => 'url',
                    ),
                    array(
                        'name'    => 'Link-URL incl. http://',
                        'id'      => 'link',
                        'type'    => 'url',
                    ),
                    array(
                        'name'    => 'FB + Link active?',
                        'id'      => 'linked',
                        'type'    => 'checkbox',
                    ),
                    array(
                        'name'    => 'Price (only Google KG events)',
                        'id'      => 'price',
                        'type'    => 'text',
                    ),
                ),
            ),
            
            array(
                'name'  => 'Preview Box',
                'desc'  => 'Fixed preview boxes for content',
                'id'    => $prefix . 'group',
                'type'  => 'group',
                'clone' => true,
                
                // List of child fields
                'fields' => array(
                    array(
                        'name'    => 'Date',
                        'id'      => 'date',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Image-URL (350x180)',
                        'id'      => 'url',
                        'type'    => 'url',
                    ),
                    array(
                        'name'    => 'Title',
                        'id'      => 'title',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Subtitle',
                        'id'      => 'subtitle',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Link-URL incl. http://',
                        'id'      => 'link',
                        'type'    => 'text',
                    ),
                    array(
                        'name'    => 'Open link in new tab',
                        'id'      => 'target',
                        'type'    => 'checkbox',
                    ),
                ),
            ),
            
        )
    );

    return $meta_boxes;
}

ah! thanks so much!