Dashboard of subfolder-site does not work

Hello,

today I changed a wordpress-site to a subfolder-multisite and I cannot go to the dashboard of the subsites, I get a redirecting error, which comes on the url https://schaltbarefolie.de/schaltbarespiegeltuere/wp-admin/. I find the site on https://schaltbarefolie.de/schaltbarespiegeltuere/, and because I can nothing do at the backend the layout is terrible and there is no content as well. The main site https://schaltbarefolie.de/ and its backend https://schaltbarefolie.de/wp-admin/ works well.

In config.php I added the following code:

define( 'WP_ALLOW_MULTISITE', true );
define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'schaltbarefolie.de');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

and my .htaccess is

# BEGIN WordPress
<IfModule mod_rewrite>
RewriteEngine On
RewriteBase /schaltbarefolie.de/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /schaltbarefolie.de/index.php [L]
</IfModule>

# END WordPress

I think, there is something wrong with the .htaccess, but I cannot find out what it is.