I had this problem with a multisite setup. On my secondary site load-scripts.php returned a 404 error.
the following line in wp-config.php fixed it for me:
define( 'CONCATENATE_SCRIPTS', false ); # worked for me
my .htaccess file:
# BEGIN manual WordPress Multisite
# The directives (lines) between `BEGIN WordPress` and `END WordPress` are
# dynamically generated, and should only be modified via WordPress filters.
# Any changes to the directives between these markers will be overwritten.
<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).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>
# END WordPress
my wp-config.php additional settings:
define( 'WP_ALLOW_MULTISITE', true );
define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );
$base="https://wordpress.stackexchange.com/";
define( 'DOMAIN_CURRENT_SITE', 'localhost' );
define( 'PATH_CURRENT_SITE', "https://wordpress.stackexchange.com/" );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );
define('COOKIE_DOMAIN', '');
define( 'ADMIN_COOKIE_PATH', "https://wordpress.stackexchange.com/" );
define( 'COOKIEPATH', "https://wordpress.stackexchange.com/" );
define( 'SITECOOKIEPATH', "https://wordpress.stackexchange.com/" );
define( 'CONCATENATE_SCRIPTS', false );
/* That's all, stop editing! Happy publishing. */