This has happened to me in the past, and whether it makes sense or not, I found that the problem seemed to be fixed when I didn’t use the latest version of jQuery on the admin screen.
For my sites, I would always use the latest version of jQuery by deregistering the jQuery that comes with WordPress and then enqueing the latest version of jQuery. Sometimes, I would have this “white text” problem in the WYSIWYG editor.
To fix it, I added if(!is_admin()) to my jQuery deregister/enqueue code so the admin dashboard still used the old jQuery and voila! Fixed.
If you’re not loading your own jQuery, also check to see if any plugins are using the latest jQuery. If so, you may need to edit the plugin.
Hope that helps!