Fresh Multisite, Sub-Domains working, Sub-directories not working

As i already stated in the title,

i installed the 4.4.2 version of wordpress, went through the process of creating a network, everything “went well” until i created a new page.

my .htaccess file looks like that:

RewriteEngine On
RewriteBase /sites/
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).*) sites/$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ sites/$2 [L]
RewriteRule . index.php [L]

my wp-config.php right before /* That’s all, stop editing! Happy blogging. */ looks like that:

/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );
define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'pbc.eu');
define('PATH_CURRENT_SITE', '/sites/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

ok, i create a new site, wanna go to the cockpit error 404 wp-admin site doesnt exist..

aight, in my wp-config.php file i set define('SUBDOMAIN_INSTALL', true);

deleted the page i created, created it again. it’s working. everything is perfect.

but i dont want it to be a sub-domain, i want it to be a sub-directory

Please help.

PS: I’m totally new to this multisite “thing” – i installed it yesterday

PPS: i went through the forum, but i didn’t find THE problem that i have, similar ones but not this one