White "Screen" after activating Child-Theme

Hi

After I activated my Child-Theme of “twentyfifteen” I get a complete white Browser (no HTML is loaded) and because of this I’m not able to login to the Backend anymore.

My Website is located at mydomain.ch/hk.
The style.css of the Child-Theme contains this:

/*
Theme Name: twentyfifteen-child
Description: Mein Design von twentyfifteen
Template: twentyfifteen
*/

The functions.php of the Child-Theme contain this:

<?php

// Enqueue-Methode Childtheme Methode gem. https://journalismus-plus.de/optimieren/anlegen-eines-child-themes-mit-der-enqueue-methode/
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array('parent-style')
    );
}

// Soll verhindern dass die Enqueue-Styles 2x geladen werden
function theme_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
}
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );

The Child-Theme is located at mydomain.ch/hk/wp-content/themes/twentyfifteen-child
The Parent-Theme is located at mydomain.ch/hk/wp-content/themes/twentyfifteen

Question:
How can I activate my Parent-Theme again to be able to continue working ?

Edit:
I just found out how to get back to the Backend (just rename the Child-Theme Folder).
BUT – What’s wrong with my Child-Theme ? Why does it produce a white Screen ?

I appreciate your help very mutch !
Thanks in advance !

Kind regards
Alain