register_sidebar fails when using a include within functions.php

You’ve made a mistake in your widgets.php file, most probably forgetting to add an opening </php tag at the start of your file. Also, I forgot one more thing, require_once("includes/widgets.php"); should not be in any function or hook

Just for the sake of interes, you can cut down on code if you register many sidebars by using a foreach loop. Here is how I register my sidebars in my theme

<?php
function pietergoosen_widgets_init() {
$mainWidgets = array (
                'sidebar-10' => array (
                    __( 'Main Sidebar', 'pietergoosen' ) => __( 'Main Sidebar for the website pages', 'pietergoosen' ),
                ),
                'sidebar-11' => array (
                    __( 'Content Sidebar', 'pietergoosen' ) => __( 'Additional sidebar on the right of pages', 'pietergoosen' ),
                ),
                // Sidebars 12, 13 and 14 not redundant as from twentyfourteen
                'sidebar-15' => array (   // Used in footer-functions.php   
                    __( 'Footer Widget One', 'pietergoosen' ) => __( 'Footer widgets', 'pietergoosen' ),
                ),
                'sidebar-16' => array (   // Used in footer-functions.php   
                    __( 'Footer Widget Two', 'pietergoosen' ) => __( 'Footer widgets', 'pietergoosen' ),
                ),
                'sidebar-17' => array (   // Used in footer-functions.php   
                    __( 'Footer Widget Three', 'pietergoosen' ) => __( 'Footer widgets', 'pietergoosen' ),
                ),
                'sidebar-18' => array (   // Used in footer-functions.php  
                    __( 'Footer Widget Four', 'pietergoosen' ) => __( 'Footer widgets', 'pietergoosen' ),
                ),
                'sidebar-19' => array (   // Used in content-functions.php   
                    __( 'Inside first post widget for advertising', 'pietergoosen' ) => __( 'Inside first post widget', 'pietergoosen' ),
                ),
                'sidebar-20' => array (   // Used in content-functions.php    
                    __( 'After first post widget', 'pietergoosen' ) => __( 'After first post widget for ads', 'pietergoosen' ),
                )
            );

foreach ( $mainWidgets as $mainId => $mainWidget ) {
foreach ($mainWidget as $mainWidgetname => $mainWidgetdescription)
    {
    register_sidebar(
        array (
                'name'          => __( $mainWidgetname, 'pietergoosen' ),
                'id'            =>  $mainId,
                'description'   => __( $mainWidgetdescription, 'pietergoosen' ),
                'before_widget' => '<aside id="%1$s" class="widget %2$s">',
                'after_widget'  => '</aside>',
                'before_title'  => '<h1 class="widget-title">',
                'after_title'   => '</h1>',
        ));
    }
}
}

add_action( 'widgets_init', 'pietergoosen_widgets_init' );