To fix this issue, in the function register_user_taxonomy() you have to add the show_ui-option to the register_taxonomy()-call and set it to true.
Here is the code (user-groups.php):
function register_user_taxonomy() {
register_taxonomy(
'user-group',
'user',
array(
'public' => false,
'labels' => array(
'name' => __( 'User Groups' ),
'singular_name' => __( 'Group' ),
'menu_name' => __( 'User Groups' ),
'search_items' => __( 'Search Groups' ),
'popular_items' => __( 'Popular Groups' ),
'all_items' => __( 'All User Groups' ),
'edit_item' => __( 'Edit User Group' ),
'update_item' => __( 'Update User Group' ),
'add_new_item' => __( 'Add New User Group' ),
'new_item_name' => __( 'New User Group Name' ),
'separate_items_with_commas' => __( 'Separate user groups with commas' ),
'add_or_remove_items' => __( 'Add or remove user groups' ),
'choose_from_most_used' => __( 'Choose from the most popular user groups' ),
),
'rewrite' => false,
'capabilities' => array(
'manage_terms' => 'edit_users', // Using 'edit_users' cap to keep this simple.
'edit_terms' => 'edit_users',
'delete_terms' => 'edit_users',
'assign_terms' => 'read',
),
'update_count_callback' => array(&$this, 'update_user_group_count'), // Use a custom function to update the count. If not working, use _update_post_term_count
'show_ui' => true
)
);
}