I had a similar problem (I coludn’t even access my admin dashboard) and this site helped me.
In the end it turned out I didn’t have this variable set in my wp-config.php:
define('MULTISITE', true);
It’s funny, because I think I copied everything WP gave me during the multisite setup, but it could be I skipped this line, because I already had define('WP_ALLOW_MULTISITE', true);
inside.