Erstmal Danke für die schnelle Reaktion.
Ich habe WP über SiteGround.com mit 1-Click installiert.
Soweit lief alles gut.

### wp-core ###

version: 5.6
site_language: de_DE
user_language: de_DE
timezone: +00:00
permalink: /blog/%postname%/
https_status: true
multisite: true
user_registration: false
blog_public: 1
default_comment_status: open
environment_type: production
user_count: 1
site_count: 1
network_count: 1
dotorg_communication: true

### wp-active-theme ###

name: Twenty Twenty-One (twentytwentyone)
version: 1.0 (latest version: 1.1)
author: the WordPress team
author_website: https://wordpress.org/
parent_theme: none
theme_features: core-block-patterns, automatic-feed-links, title-tag, post-formats, post-thumbnails, menus, html5, custom-logo, customize-selective-refresh-widgets, wp-block-styles, align-wide, editor-styles, editor-style, editor-font-sizes, custom-background, editor-color-palette, editor-gradient-presets, responsive-embeds, custom-line-height, experimental-link-color, custom-spacing, custom-units, widgets
theme_path: /home/customer/www/lowkeymalerei.de/public_html/wp-content/themes/twentytwentyone
auto_update: Deaktiviert

### wp-themes-inactive (2) ###

Twenty Nineteen: version: 1.8, author: the WordPress team (latest version: 1.9), Automatische Aktualisierungen deaktiviert
Twenty Twenty: version: 1.6, author: the WordPress team, Automatische Aktualisierungen deaktiviert

### wp-plugins-active (1) ###

SG Optimizer: version: 5.7.12, author: SiteGround, Automatische Aktualisierungen deaktiviert

### wp-media ###

image_editor: WP_Image_Editor_GD
imagick_module_version: Nicht verfügbar
imagemagick_version: Nicht verfügbar
file_uploads: File uploads is turned off
post_max_size: 256M
upload_max_filesize: 256M
max_effective_size: 256 MB
max_file_uploads: 20
gd_version: 2.2.5
ghostscript_version: 9.27

### wp-server ###

server_architecture: Linux 3.12.18-clouder0 x86_64
httpd_software: Apache
php_version: 7.3.26 64bit
php_sapi: cgi-fcgi
max_input_variables: 3000
time_limit: 120
memory_limit: 768M
max_input_time: 120
upload_max_filesize: 256M
php_post_max_size: 256M
curl_version: 7.66.0 OpenSSL/1.1.1g-fips
suhosin: false
imagick_availability: false
pretty_permalinks: true
htaccess_extra_rules: false

### wp-database ###

extension: mysqli
server_version: 5.7.26-29-log
client_version: mysqlnd 5.0.12-dev - 20150407 - $Id: 7cc7cc96e675f6d72e5cf0f267f48e167c2abb23 $

### wp-constants ###

WP_HOME: undefined
WP_SITEURL: undefined
WP_CONTENT_DIR: /home/customer/www/lowkeymalerei.de/public_html/wp-content
WP_PLUGIN_DIR: /home/customer/www/lowkeymalerei.de/public_html/wp-content/plugins
WP_MAX_MEMORY_LIMIT: 768M
WP_DEBUG: false
WP_DEBUG_DISPLAY: true
WP_DEBUG_LOG: false
SCRIPT_DEBUG: false
WP_CACHE: false
CONCATENATE_SCRIPTS: undefined
COMPRESS_SCRIPTS: undefined
COMPRESS_CSS: undefined
WP_LOCAL_DEV: undefined
DB_CHARSET: utf8
DB_COLLATE: undefined

### wp-filesystem ###

wordpress: writable
wp-content: writable
uploads: writable
plugins: writable
themes: writable