"Error establishing a database connection" when enabling multisite

Hi,
i have this strange issue:
I enabled the multisite and everything was working fine, then i had to revert back to a backup taken just before enabling it (by the plesk panel), and now if i try to enable it again following the same steps, i get the database connection error.
It’s a subdirectory install, and if i go to mysite.com/wordpress it works fine, but i can’t get rid of the error when on mysite.com.
Before enabling the multisite the site was up for few months, giving no problems when accessing it from mysite.com.

The second time i enabled the multisite it skipped the part where asks for the network name and other things, i think the restore of the backup didn’t erased this informations, and god knows what else…

The exact moment when it brokes it’s when i paste the provided lines in the wp-config.php and .htaccess files and click on “login”.

suggestions on where to investigate?

here is my .htaccess (in the root dir):

# BEGIN WordPress
<IfModule mod_rewrite.c>
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).*) wordpress/$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ wordpress/$2 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress

wp-config.php(just the relevant part, after the db connection configuration, which is 100% correct):

define('WP_DEBUG', false);

define('WP_HOME','https://mysite.com/wordpress');
define('WP_SITEURL','https://mysite.com/');

define( 'WP_ALLOW_MULTISITE', true );

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'mysite.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 ('FS_METHOD', 'direct');

//--- disable auto upgrade
define( 'AUTOMATIC_UPDATER_DISABLED', true );

index.php (in the root dir):

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wordpress/wp-blog-header.php' );

and this are the first lines of functions.php:

<?php
update_option('siteurl','https://mysite.com/');
update_option('home','https://mysite.com/wordpress');