After moving a WordPress site from a development environment area to the production (which involved the domain change), the wp-admin area doesn't load any CSS Styles or Scripts. Basically, the admin section is completely broken. The website front-end is not affected and working fine.
Browser Dev Console shows 403 (Forbidden) errors loading files wp-admin/load-styles.php and wp-admin/load-styles.php.
Failed to load resource: the server responded with a status of 403.
The issue is not client related, different browsers and PCs are affected.
The short-term resolution was to add the following line to wp-config.php:
Another option was to add:
define( 'SCRIPT_DEBUG', true );
Which also resolved the issue.
The long-term solution was to do... absolutely nothing... I assume this was some sort of WordPress cashing issue. About 5 hours after the migration, everything started working correctly without the above modifications.