hbwiensued

Danke für die Anregungen.
Ich habe das letzte funktionierende Backup meines Providers heruntergeladen. Dann habe ich die aktuelle Version mit dem Backup verglichen. Zuerst die Verzeichnisse und danach die Dateien.
Die Datei wp-config.php hatte Unterschiede bei der Datenbank. Durch Ersetzen von wp-config.php aus dem Backup war der Fehler behoben.
Danke an alle die mir geholfen haben.

Hier ist der Website-Bericht:

wp-core

version: 5.4.2
site_language: de_DE
user_language: de_DE
timezone: Europe/Berlin
permalink: /%category%/%postname%/
https_status: true
user_registration: 0
default_comment_status: open
multisite: false
user_count: 2
dotorg_communication: true wp-paths-sizes

wordpress_path: /data/web/e97283/html
wordpress_size: 26,62 GB (28582447110 bytes)
uploads_path: /data/web/e97283/html/wp-content/uploads
uploads_size: 10,25 GB (11005922642 bytes)
themes_path: /data/web/e97283/html/wp-content/themes
themes_size: 34,29 MB (35952875 bytes)
plugins_path: /data/web/e97283/html/wp-content/plugins
plugins_size: 158,40 MB (166095432 bytes)
database_size: 59,50 MB (62390021 bytes)
total_size: 37,12 GB (39852808080 bytes) wp-active-theme

name: Native Church (NativeChurch)
version: 3.0
author: imithemes
author_website: http://www.imithemes.com
parent_theme: none
theme_features: post-thumbnails, title-tag, automatic-feed-links, post-formats, woocommerce, wc-product-gallery-zoom, wc-product-gallery-lightbox, wc-product-gallery-slider, menus, widgets, editor-style
theme_path: /data/web/e97283/html/wp-content/themes/NativeChurch wp-themes-inactive (1)

Twenty Seventeen: version: 3.7, author: Das WordPress-Team wp-plugins-active (34)

Borlabs Cookie - Cookie Opt-in: version: 2.2.68, author: Borlabs GmbH
Breadcrumb NavXT: version: 7.2.0, author: John Havlik
CC Child Pages: version: 1.45, author: Caterham Computing
Column Shortcodes: version: 1.0.1, author: Codepress
Combo Blocks: version: 2.2.88, author: PickPlugins (latest version: 2.2.89)
Coming Soon Page, Maintenance Mode, Landing Pages & WordPress Website Builder by SeedProd: version: 6.18.5, author: SeedProd
Contact Form 7: version: 5.3.2, author: Takayuki Miyoshi
Disable Gutenberg: version: 3.1.2, author: Jeff Starr
Easy Updates Manager: version: 9.0.18, author: Easy Updates Manager Team
Enable jQuery Migrate Helper: version: 1.4.0, author: The WordPress Team
Gwolle Guestbook: version: 4.6.2, author: Marcel Pol
iPray: version: 1.5, author: imithemes
Loco Translate: version: 2.6.11, author: Tim Whitlock
MaxButtons: version: 9.7.8, author: Max Foundry
MC4WP: Mailchimp for WordPress: version: 4.9.14, author: ibericode (latest version: 4.9.15)
Media Sync: version: 1.4.6, author: Media Sync Team
Newsletter: version: 8.4.7, author: Stefano Lissa & The Newsletter Team
Newsletter - Addons Manager and Support: version: 1.3.1, author: The Newsletter Team
Newsletter - Archive: version: 4.1.5, author: The Newsletter Team
PDF Embedder: version: 4.6.4, author: WP PDF Embedder Team
PHP Compatibility Checker: version: 1.5.2, author: WP Engine
Pojo Sidebars: version: 1.0.3, author: Pojo Team
ResponsiveVoice Text To Speech: version: 1.7.13, author: ResponsiveVoice
Simple 301 Redirects: version: 2.0.10, author: WPDeveloper
Social Icons Widget: version: 16.07, author: Daniel Nisbet
UpdraftPlus - Backup/Restore: version: 1.24.4, author: UpdraftPlus.Com, DavidAnderson
WP-Optimize - Clean, Compress, Cache: version: 3.5.0, author: David Anderson, Ruhani Rabin, Team Updraft
WP Downgrade | Specific Core Version: version: 1.2.6, author: Reisetiger
WP Edit Pro: version: 4.4.4, author: Josh Lobe
WP phpMyAdmin: version: 5.2.0.7, author: Puvox.software
WP RSS Aggregator: version: 4.23.12, author: RebelCode
WP Sitemap Page: version: 1.9.4, author: Tony Archambeau
WP Twingle: version: 1.4, author: twingle GmbH
Yoast Duplicate Post: version: 3.2.6, author: Enrico Battocchi & Team Yoast wp-media

image_editor: WP_Image_Editor_Imagick
imagick_module_version: 1690
imagemagick_version: ImageMagick 6.9.10-23 Q16 x86_64 20190101 https://imagemagick.org
imagick_limits:
imagick::RESOURCETYPE_AREA: 122 MB
imagick::RESOURCETYPE_DISK: 1073741824
imagick::RESOURCETYPE_FILE: 786432
imagick::RESOURCETYPE_MAP: 512 MB
imagick::RESOURCETYPE_MEMORY: 256 MB
imagick::RESOURCETYPE_THREAD: 1
gd_version: 2.3.3
ghostscript_version: 9.27 wp-server

server_architecture: Linux 4.19.0-25-amd64 x86_64
httpd_software: Apache
php_version: 7.4.33 64bit
php_sapi: fpm-fcgi
max_input_variables: 2000
time_limit: 300
memory_limit: 256M
max_input_time: -1
upload_max_size: 256M
php_post_max_size: 256M
curl_version: 7.64.0 OpenSSL/1.1.1n
suhosin: false
imagick_availability: true
htaccess_extra_rules: true wp-database

extension: mysqli
server_version: 5.7.36-39-log
client_version: mysqlnd 7.4.33 wp-constants

WP_HOME: undefined
WP_SITEURL: undefined
WP_CONTENT_DIR: /data/web/e97283/html/wp-content
WP_PLUGIN_DIR: /data/web/e97283/html/wp-content/plugins
WP_MAX_MEMORY_LIMIT: 256M
WP_DEBUG: false
WP_DEBUG_DISPLAY: true
WP_DEBUG_LOG: false
SCRIPT_DEBUG: false
WP_CACHE: false
CONCATENATE_SCRIPTS: false
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