Check your Nginx config. It may be blocking some files. In my case the server was blocking TinyMCE, and I worked around it by adding this to my theme to avoid the ?wp-mce-* query string that some WAFs block:
// TinyMCE: avoid ?wp-mce-**** query that some WAFs block add_filter('tiny_mce_before_init', function ($init) { $init['cache_suffix'] = 'v=1'; return $init; });
This solved it for me.
