Rule 1: WordPress rules ALWAYS go at the bottom.
# mod_deflate (gzip) aktivieren
<FilesMatch "\\.(js|css|html|htm|php|xml)$">
SetOutputFilter DEFLATE
</FilesMatch>
# ExpiresHeader: verhindert bedingte GET-Anfragen
<IfModule mod_expires.c>
ExpiresActive on
ExpiresDefault "access plus 35 days"
</IfModule>
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} !(Chrome\/[0-8]|Android\s[0-3])\.
RewriteCond %{HTTP_USER_AGENT} Chrome [OR]
RewriteCond %{HTTP_ACCEPT} image/webp
RewriteCond %{DOCUMENT_ROOT}/$1.webp -f
RewriteRule ^(wp-content/uploads.+)\.(jpe?g|png)$ $1.webp [L,T=image/webp]
</IfModule>
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ - [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 . /wordpress/index.php [L]
</IfModule>
# END WordPress
See if that helps.