I recently activated multisite and received my code to add to wp-config.php.

WordPress wanted to force me to use subdomains.

I wanted to use subdirectories, so I used:

define( 'WP_ALLOW_MULTISITE', true);
define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );
define( 'DOMAIN_CURRENT_SITE', 'www.example.com' );
define( 'PATH_CURRENT_SITE', "https://wordpress.stackexchange.com/" );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );

I then used the .htaccess code from this answer:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) /wp-includes/ms-files.php?file=$2 [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).*) /$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ /$2 [L]
RewriteRule . index.php [L]

However, I still receive a redirect loop when trying to load www.example.com/au/wp-admin/:

This page isn’t working 
www.example.com redirected you too many times.
Try clearing your cookies.
ERR_TOO_MANY_REDIRECTS

I have cleared my cookies and the issue remains.

Help appreciated.

EDIT

I created a blank local MU subdirectory site and copied its .htaccess:

RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
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).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

but the error remains.