Unable to enter Admin Panel/Dashboard

I’ve just enabled a multisite network as written in step 4 in https://codex.www.ads-software.com/Create_A_Network.

After clearing cache I tried to login via domain.com/wp-login/ but got redirected to the root. Also when I go to domain.com/wp-admin/ I got instantly redirected to the root.

I’ve tried to include some code in wp-config.php

define('ADMIN_COOKIE_PATH', '/');
define('COOKIE_DOMAIN', '');
define('COOKIEPATH', '');
define('SITECOOKIEPATH', '');

from https://tommcfarlin.com/resolving-the-wordpress-multisite-redirect-loop/, but no luck.

My hosting provider pointed me in the same direction and tells me the error log does not show any error, so they can not help me solve this problem…

Any ideas?