Ok, thank you so much for your fast reply.
Call me stupid but now something very strange happened. When I want to login I'm just getting redirected to the wp-signup.php page.
If I delete the text in the wp-config.php (define('MULTISITE', true); etc) and also the text in the .htcaccess I can log in - but as a user without access to the dashboard or anything else.
What did I do?! I don't dare to touch any other folders, I don't want to make it worse. I've lost all my privileges as a superadmin, even as an admin as I can't change anything on my website.
I wanted my host to reset my website to a previous date but they're not responding... Have no idea how to solve this...