WP-Admin not working properly at Wordpress multisite with subdirectories

Question

I have recently installed a Wordpress Multisite Blog on my domain.

Everything is working fine, except for wp-admin of the subsites:

I can access wp-admin of a subsite, but css is not loaded and when I try to click on a navigation link I get the wordpress "File not found" error.

That is because it links to e.g. domain.com/subsite/wp-admin/post-new.php - which does not exist. (domain.com/wp-admin/post-new.php exists)

I also think that css does not work, domain.com/subsite/wp-admin/load-styles.php and load-scripts.php cannot be found

Here is one of the errors I get in the chrome developer tools when visiting /subsite/wp-admin

I am using the standard .htaccess that is pasted everywhere:

    # BEGIN WordPress
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]
# END WordPress

I guess the error must be here, right?

Here is wp-config - just in case:

<?php

 define('WP_ALLOW_MULTISITE', true);

// ** MySQL settings - You can get this info from your web host ** //
/** The name of the database for WordPress */
define('DB_NAME', 'XXX');

/** MySQL database username */
define('DB_USER', 'XXX');

/** MySQL database password */
define('DB_PASSWORD', 'XXX');

/** MySQL hostname */
define('DB_HOST', 'XXX');

/** Database Charset to use in creating database tables. */
define('DB_CHARSET', 'utf8');

/** The Database Collate type. Don't change this if in doubt. */
define('DB_COLLATE', '');

/**#@+
 * Authentication Unique Keys and Salts.
 *
 * Change these to different unique phrases!
 * You can generate these using the {@link https://api.wordpress.org/secret-key/1.1/salt/ WordPress.org secret-key service}
 * You can change these at any point in time to invalidate all existing cookies. This will force all users to have to log in again.
 *
 * @since 2.6.0
 */
define('AUTH_KEY', XXX);
define('SECURE_AUTH_KEY',XXX);
define('LOGGED_IN_KEY',XXX);
define('NONCE_KEY',        XXX);
define('AUTH_SALT',        XXX);
define('SECURE_AUTH_SALT', XXX);
define('LOGGED_IN_SALT',   XXX);
define('NONCE_SALT',       XXX);
/**#@-*/

/**
 * WordPress Database Table prefix.
 *
 * You can have multiple installations in one database if you give each
 * a unique prefix. Only numbers, letters, and underscores please!
 */
$table_prefix = 'XXX';

/**
 * For developers: WordPress debugging mode.
 *
 * Change this to true to enable the display of notices during development.
 * It is strongly recommended that plugin and theme developers use WP_DEBUG
 * in their development environments.
 *
 * For information on other constants that can be used for debugging,
 * visit the Codex.
 *
 * @link https://codex.wordpress.org/Debugging_in_WordPress
 */
define('WP_DEBUG', false);


define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'mydomain.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);




/* That's all, stop editing! Happy blogging. */

/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
    define('ABSPATH', dirname(__FILE__) . '/');

/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');

define( 'WP_ALLOW_MULTISITE', true );

define ('FS_METHOD', 'direct');
?>

Answer 1

If you installed WordPress Multisite starting with version 3.0 to 3.4.2, you've got the correct .htaccess file contents.

However, if you started with a newer version (3.5 or higher)—and I'm assuming you did, if you've just installed WordPress Multisite recently—your .htaccess file should look like this:

RewriteEngine On
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]

I'm assuming you're using Multisite in Subfolder mode, since in your question your URL is of the form example.com/site/wp-admin. (Edit: Also I just noticed that it's definitely Subfolder per your posted wp-config.php file.)

See the Codex page on .htaccess for details.