WordPress Multisite breaks login/logout session

We wrote a simple login sync that will automatically login users to WordPress when they login on our main website. This worked fine on a regular WordPress, but as soon as we enabled Multisite it gave us

Error establishing a database connection.

Does Multisite changes certain things to cause this? Any help would be great appreciated!

Here is the code for login sync that works with wordpress single, but breaks as soon as multisite is enabled.

require_once('../3/wp-load.php'); //Include WP core
wp_create_user( $request->getPost('username'), $request->getPost('password'), $request->getPost('email') );
$rusername = $request->getPost('username'); // Get username of loged user
$ruser = get_user_by('login', $rusername );

if ( !is_wp_error( $ruser ) )
{
    wp_clear_auth_cookie();
    wp_set_current_user ( $ruser->ID ); //Set current loged user in WP
    wp_set_auth_cookie  ( $ruser->ID ); //Set current Loged user cookie/session in WP
}