Visual Editor Missing, Server-Side Problem, How Would You Debug It?

I had a related issue and it wasn’t solved using the above answers. I lost access to the Visual Editor in the classic editor, and was stuck in HTML. When using Gutenberg, I lost access to the visual editor, could not add blocks, and could not switch from Visual to Code tabs.

The problem was caused by AWS CloudFront removing the UserAgent headers, which WP uses to determine whether or not visual editing is available.

If anyone comes here and the above answers don’t help, look into your hosting solution and see if you have a similar scenario to mine.

The fix is adjusting the way your host/AWS treats headers, like this post specifies, or you can use the following code to re-enable the flags removed by WP due to the lack of the UserAgent header:

function richedit_wp_cloudfront () {
   add_filter('user_can_richedit','__return_true');
}

add_action( 'init', 'richedit_wp_cloudfront', 9 );

The problem and solution are detailed in a few places:

• Block editor no longer working: only shows HTML
• Cannot add block for any post or user