No valid JSON request

l Thank you for your quick response. I was able to narrow down and locate the problem now. It concerns the plugin Visual-Link: https://de.wordpress.org/plugins/visual-link-preview/

Everything is displayed normally in the front of the website. In the backend, the error arises: error when loading the block. The answer is not a valid JSON answer. I have activated BBQ Firewall on the productive page: https://shop.drogist-n.de, so you can get a picture.

# Schütze wp-config.php
<Files wp-config.php>
    Order allow,deny
    Deny from all
</Files>


# Deaktiviere Directory Browse
Options -Indexes

# Server Signature deaktivieren
ServerSignature Off

# Zugriff auf install.php blockieren
<Files install.php>
    Order Allow,Deny
    Deny from all
</Files>

# Zugriff auf readme.html/liesmich.html blockieren
<FilesMatch "^(readme|liesmich)\.html?$">
    Order Allow,Deny
    Deny from all
    Satisfy All
</FilesMatch>

# Zugriff auf error_log blockieren
<Files error_log>
    Order Allow,Deny
    Deny from all
</Files>

# Deaktiviere XML-RPC
<Files xmlrpc.php>
    Order Deny,Allow
    Deny from all
</Files>

<IfModule mod_rewrite.c>
RewriteEngine On
# Hotlinking-Schutz aktivieren
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?shop\.drogist-n\.de [NC]
RewriteRule \.(jpg|jpeg|png|gif|bmp|webp|svg)$ - [NC,F,L]

RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
</IfModule>

<IfModule mod_headers.c>
Header set Connection keep-alive

Header unset ETag
FileETag None

<FilesMatch "\.(js|mjs|css|xml|gz|html|json|webmanifest)$">
    Header append Vary Accept-Encoding
</FilesMatch>

Header set Referrer-Policy "strict-origin-when-cross-origin"

Header set X-Frame-Options "SAMEORIGIN"

Header set X-Content-Type-Options "nosniff"

Header set Strict-Transport-Security "max-age=31536000"

Header set Content-Security-Policy "upgrade-insecure-requests"

Header set Permissions-Policy "geolocation=(), midi=(), sync-xhr=(), microphone=(), camera=(), magnetometer=(), gyroscope=(), speaker=(self), vibrate=(), fullscreen=(self), payment=()"
</IfModule>

<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault                              "access plus 1 month"
ExpiresByType image/jpeg                    "access plus 1 year"
ExpiresByType image/png                     "access plus 1 year"
ExpiresByType image/gif                     "access plus 1 year"
ExpiresByType image/webp                    "access plus 1 year"
ExpiresByType image/svg+xml                 "access plus 1 year"
ExpiresByType image/x-icon                  "access plus 1 week"
ExpiresByType text/css                      "access plus 1 year"
ExpiresByType application/javascript        "access plus 1 year"
ExpiresByType application/x-javascript      "access plus 1 year"
ExpiresByType application/pdf               "access plus 1 month"
ExpiresByType application/font-woff         "access plus 1 year"
ExpiresByType application/font-woff2        "access plus 1 year"
ExpiresByType application/vnd.ms-fontobject "access plus 1 year"
ExpiresByType font/ttf                      "access plus 1 year"
ExpiresByType font/otf                      "access plus 1 year"
ExpiresByType text/html                     "access plus 0 seconds"
ExpiresByType application/json              "access plus 0 seconds"
ExpiresByType application/xml               "access plus 0 seconds"
ExpiresByType text/xml                      "access plus 0 seconds"
ExpiresByType application/rss+xml           "access plus 1 hour"
</IfModule>

<IfModule mod_filter.c>
<IfModule mod_brotli.c>
        AddOutputFilterByType BROTLI_COMPRESS text/html text/plain text/xml text/css text/javascript application/javascript application/x-javascript application/json application/xml application/rss+xml image/svg+xml
    </IfModule>
    <IfModule mod_deflate.c>
        <IfModule mod_setenvif.c>
            SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png|webp|zip|gz|tgz|bz2|rar|7z)$ no-gzip dont-vary
        </IfModule>
        AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/javascript application/x-javascript application/json application/xml application/rss+xml image/svg+xml
        <IfModule mod_headers.c>
           Header append Vary User-Agent env=!dont-vary
        </IfModule>
    </IfModule>
</IfModule>

<IfModule mod_alias.c>
RedirectMatch 403 (?i)\.php\.suspected$
RedirectMatch 403 (?i)/apple-app-site-association$
RedirectMatch 403 (?i)/autodiscover/autodiscover\.xml$
</IfModule>

# BEGIN WordPress
# Die Anweisungen (Zeilen) zwischen „BEGIN WordPress“ und „END WordPress“ sind
# dynamisch generiert und sollten nur über WordPress-Filter geändert werden.
# Alle Änderungen an den Anweisungen zwischen diesen Markierungen werden überschrieben.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

Danke für deine Hilfe, es ist eine gut programmierte Firewall. Beim bearbeiten mit dem Plugin Visual-Link schalte ich die Firewall temorär ab.

Thanks for your help, it’s a well-programmed firewall. When editing with the plugin Visual-Link, I turn off the firewall in a temorary way.