WordPress 4.7.2 update on multisite causes wp-admin redirect loop

I have a website running multisite using a subdirectory setup. I have just tried updating my site to 4.7.2 and the update goes through and no errors happen during the installation, but then after the install is finished I am redirected to /wp-admin/network/about.php?updated which returns a 404 page. So then I try to go back to /wp-admin/ and I get a redirect loop error.

I have looked through at least 40 different articles, forums, and websites about WordPress multisite wp-admin redirect loop issues. I have tried everything I can possibly find to fix this issue, but nothing has worked. The main site works perfectly fine, but wp-admin is completely unaccessible.

Here are the things I have tried so far:

1. I tried clearing my cache and cookies multiple times, and the site is not using any caching plugins
2. I renamed my plugins folder to plugins.deactivate
3. Just to be extra thorough I also updated active_plugins field in the wp_options table to a:0:{}
4. I tried removing multisite definitions from my wp-config.php file, but when that had no effect I put them back
5. I confirmed my .htaccess file matches the one listed here for 3.5+ multisite – https://codex.wordpress.org/Multisite_Network_Administration#.htaccess_and_Mod_Rewrite
6. I also tried switching it to the old version, just in case, but that did nothing so I switched it back.
7. I found a forum saying that they fixed the issue by commenting out a redirect at the bottom of /wp-admin/network/admin.php so I tried that but it did nothing so I uncommented it back to normal.

Here is my .htaccess file:

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]

And here is my wp-config.php file:

<?php
/**
 * The base configuration for WordPress
 *
 * The wp-config.php creation script uses this file during the
 * installation. You don't have to use the web site, you can
 * copy this file to "wp-config.php" and fill in the values.
 *
 * This file contains the following configurations:
 *
 * * MySQL settings
 * * Secret keys
 * * Database table prefix
 * * ABSPATH
 *
 * @link https://codex.wordpress.org/Editing_wp-config.php
 *
 * @package WordPress
 */

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

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

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

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

/** 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',         '6YuB7pAuURnDjyFUcb3Q3rVrWiNk6K872AyD1pP2t09WPxY09Z3BVxaSFWmQxYcM');
define('SECURE_AUTH_KEY',  'FIoVpmtRhYJNWrE2G689bp2GinApBgY47OVVxrZiS4Y134lqLYIbvf57PIqSoQTX');
define('LOGGED_IN_KEY',    '8zC1jLLeAe2DH1E4k7KwXpLlkKaZEzVkDqOjx4z6csRERQhsqYvdQsroXbqvR7MG');
define('NONCE_KEY',        'Vcz1A0I1e37bksIaA5wFWQBHvJtt6zRwzmQDAlwUOW2Eo21Mn23tUldujrbwtv6s');
define('AUTH_SALT',        'ng5aCZQQ1bHQ1YkO1QlZwdcLp38yXAHssnEtofvqCL67PeVQUiFNh7Ns1UkSjMai');
define('SECURE_AUTH_SALT', 'ZP0xk8ldQc0OftLgpADR4toNPCN4sLEFuSTaRWTmGBu8dyEs1yXfwEq8Gag00AWi');
define('LOGGED_IN_SALT',   'VKmGsDjfILr5HeEXJuL7WlQ0UUcRvZQrT5Z47cR7mied28FvREetf0pvxwSE8M7l');
define('NONCE_SALT',       'JexGduB5BSCxcgNbTYEsfaKudzbcK9PzFaRgkMZUUlFy4GoQjUSx1i2MEcK8Byfv');

/**
 * Other customizations.
 */
define('FS_METHOD','direct');define('FS_CHMOD_DIR',0777);define('FS_CHMOD_FILE',0666);
define('WP_TEMP_DIR',dirname(__FILE__).'/wp-content/uploads');

/**
 * Turn off automatic updates since these are managed upstream.
 */
define('AUTOMATIC_UPDATER_DISABLED', true);

/**#@-*/

/**
 * 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  = 'kl9t_';

/**
 * 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('WP_ALLOW_MULTISITE', true);
define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', $_SERVER['HTTP_HOST']);
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');