Subdirectory Multisite Network: Subdirectory Dashboard and Website not working

After setting up a subdirectory multisite network from scratch (empty content), everything looks fine at first.

Also adding a new website to the multisite network seems to work as expected. The new site (chi-gong.net/en) is listed in the Sites page in WP as it should be.

However, when I try to access the dashboard of the subdirectory site (chi-gong.net/en), I get an error message indicating too many redirects.

When I access the new website, what I get looks like the Dashboard for this site, not the site itself.

I suppose this problem is caused by the entries in the .htaccess file which looks like this:

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# START IONOS Performance Caching
# IONOS Caching Snippet v2
<IfModule mod_setenvif.c>
SetEnvIf REQUEST_METHOD “^(?!GET).*$” INITIAL_REQUEST_METHOD=NOGET
</IfModule>

<IfModule mod_rewrite.c>
# ENGINE ON
RewriteEngine on

# set hostname directory
RewriteCond %{HTTPS} on
RewriteRule .* – [E=IONOS_PERFORMANCE_HOST:https-%{HTTP_HOST}]

RewriteCond %{HTTPS} off
RewriteRule .* – [E=IONOS_PERFORMANCE_HOST:%{HTTP_HOST}]

# set subdirectory
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_METHOD} GET
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteCond %{REQUEST_FILENAME} !.(gif|jpg|png|jpeg|css|xml|txt|js|php|scss|webp|mp3|avi|wav|mp4|mov)$ [NC]
RewriteRule .* – [E=IONOS_PERFORMANCE_DIR:%{REQUEST_URI}/]

RewriteCond %{REQUEST_URI} /$
RewriteRule .* – [E=IONOS_PERFORMANCE_DIR:%{REQUEST_URI}]

RewriteCond %{REQUEST_URI} ^$
RewriteRule .* – [E=IONOS_PERFORMANCE_DIR:/]

# gzip
RewriteRule .* – [E=IONOS_PERFORMANCE_SUFFIX:]
<IfModule mod_mime.c>
RewriteCond %{HTTP:Accept-Encoding} gzip
RewriteRule .* – [E=IONOS_PERFORMANCE_SUFFIX:.gz]
AddType text/html .gz
AddEncoding gzip .gz
</IfModule>

# Main Rules
RewriteCond %{HTTP_ACCEPT} .*text/html.*
RewriteCond %{ENV:INITIAL_REQUEST_METHOD} ^$
RewriteCond %{ENV:REDIRECT_INITIAL_REQUEST_METHOD} ^$
RewriteCond %{QUERY_STRING} ^$
RewriteCond %{REQUEST_URI} !^/(wp-admin|wp-content/cache)/.*
RewriteCond %{HTTP_COOKIE} !(wp-postpass|wordpress_logged_in|comment_author)_
RewriteCond /homepages/35/d26846717/htdocs/clickandbuilds/ChiKungWPTestsite/wp-content/cache/ionos-performance/%{ENV:IONOS_PERFORMANCE_HOST}%{ENV:IONOS_PERFORMANCE_DIR}index.html%{ENV:IONOS_PERFORMANCE_SUFFIX} -f
RewriteRule ^(.*) /wp-content/cache/ionos-performance/%{ENV:IONOS_PERFORMANCE_HOST}%{ENV:IONOS_PERFORMANCE_DIR}index.html%{ENV:IONOS_PERFORMANCE_SUFFIX} [L]
</IfModule>
# END IONOS Performance Caching

<IfModule mod_dir.c>
DirectoryIndex index.php Default.htm index.html home.html welcome.html default.html index.htm home.htm welcome.htm default.htm index.php3 index.shtml home.shtml index.cgi home.cgi home.wml index.wml index.php4 home.php4 index.php5 index.php6 baustelle.html Index.html Index.htm
</IfModule>

<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE text/cache-manifest
AddOutputFilterByType DEFLATE text/javascript
AddOutputFilterByType DEFLATE text/vcard
AddOutputFilterByType DEFLATE text/vnd.rim.location.xloc
AddOutputFilterByType DEFLATE text/vtt
AddOutputFilterByType DEFLATE text/x-component
AddOutputFilterByType DEFLATE text/x-cross-domain-policy
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/javascript
AddOutputFilterByType DEFLATE application/x-javascript
AddOutputFilterByType DEFLATE application/json
AddOutputFilterByType DEFLATE application/ld+json
AddOutputFilterByType DEFLATE application/atom+xml
AddOutputFilterByType DEFLATE application/manifest+json
AddOutputFilterByType DEFLATE application/rdf+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/schema+json
AddOutputFilterByType DEFLATE application/vnd.geo+json
AddOutputFilterByType DEFLATE application/vnd.ms-fontobject
AddOutputFilterByType DEFLATE application/x-font-ttf
AddOutputFilterByType DEFLATE application/x-javascript
AddOutputFilterByType DEFLATE application/x-web-app-manifest+json
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE font/eot
AddOutputFilterByType DEFLATE font/opentype
AddOutputFilterByType DEFLATE image/bmp
AddOutputFilterByType DEFLATE image/svg+xml
AddOutputFilterByType DEFLATE image/vnd.microsoft.icon
AddOutputFilterByType DEFLATE image/x-icon
</IfModule>

<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType text/css A2419200
ExpiresByType text/x-component A2419200
ExpiresByType application/x-javascript A2419200
ExpiresByType application/javascript A2419200
ExpiresByType text/javascript A2419200
ExpiresByType text/x-js A2419200
ExpiresByType text/html A3600
ExpiresByType text/richtext A3600
ExpiresByType image/svg+xml A3600
ExpiresByType text/plain A3600
ExpiresByType text/xsd A3600
ExpiresByType text/xsl A3600
ExpiresByType text/xml A3600
ExpiresByType video/asf A2419200
ExpiresByType video/avi A2419200
ExpiresByType image/bmp A2419200
ExpiresByType application/java A2419200
ExpiresByType video/divx A2419200
ExpiresByType application/msword A2419200
ExpiresByType application/vnd.ms-fontobject A2419200
ExpiresByType application/x-msdownload A2419200
ExpiresByType image/gif A2419200
ExpiresByType application/x-gzip A2419200
ExpiresByType image/x-icon A2419200
ExpiresByType image/jpeg A2419200
ExpiresByType application/json A2419200
ExpiresByType application/vnd.ms-access A2419200
ExpiresByType audio/midi A2419200
ExpiresByType video/quicktime A2419200
ExpiresByType audio/mpeg A2419200
ExpiresByType video/mp4 A2419200
ExpiresByType video/mpeg A2419200
ExpiresByType application/vnd.ms-project A2419200
ExpiresByType application/x-font-otf A2419200
ExpiresByType application/vnd.ms-opentype A2419200
ExpiresByType application/vnd.oasis.opendocument.database A2419200
ExpiresByType application/vnd.oasis.opendocument.chart A2419200
ExpiresByType application/vnd.oasis.opendocument.formula A2419200
ExpiresByType application/vnd.oasis.opendocument.graphics A2419200
ExpiresByType application/vnd.oasis.opendocument.presentation A2419200
ExpiresByType application/vnd.oasis.opendocument.spreadsheet A2419200
ExpiresByType application/vnd.oasis.opendocument.text A2419200
ExpiresByType audio/ogg A2419200
ExpiresByType application/pdf A2419200
ExpiresByType image/png A2419200
ExpiresByType application/vnd.ms-powerpoint A2419200
ExpiresByType audio/x-realaudio A2419200
ExpiresByType image/svg+xml A2419200
ExpiresByType application/x-shockwave-flash A2419200
ExpiresByType application/x-tar A2419200
ExpiresByType image/tiff A2419200
ExpiresByType application/x-font-ttf A2419200
ExpiresByType application/vnd.ms-opentype A2419200
ExpiresByType audio/wav A2419200
ExpiresByType audio/wma A2419200
ExpiresByType application/vnd.ms-write A2419200
ExpiresByType application/font-woff A2419200
ExpiresByType application/vnd.ms-excel A2419200
ExpiresByType application/zip A2419200
</IfModule>

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

AddHandler x-mapp-php5.5 .php

# BEGIN WordPress
# The directives (lines) between “BEGIN WordPress” and “END WordPress” are
# dynamically generated, and should only be modified via WordPress filters.
# Any changes to the directives between these markers will be overwritten.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* – [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ – [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ – [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress`
-.-

Thank you very much in advance!

The page I need help with: [log in to see the link]