Here are a couple of things that may solve the problem of a blank TinyMCE HTML editor in WordPress:-

1. Reinstall WordPress via WordPress Dashboard 'Updates' - click on 'Re-install Now' button

2. Search for a rogue .htaccess file in wp-includes folder and delete it

3.  Add the following to your wp-config.php file:-

In our case option 3 solved the problem. Let us know how you solved the same issue if any other than the above!