Visual editor blank (not even white text) after upgrade to 4.2.1

Since upgrading to 4.2.1, the visual editor displays the toolbar, but nothing in the editor window. The cursor is in the upper-left of the edit window, and attempting to highlight the text doesn’t work, because it isn’t white text – it’s no text at all. It does show the word count at the bottom, so it seems to “know” there’s supposed to be text. The text editor does work, but this is a different variation of the blank editor problem other people seem to be having.

I’ve tested on both PC and Mac, using the latest version of Firefox, Safari, Chrome, Internet Explorer, and Opera. It’s the same on all platforms and browsers.

What I’ve tried:

1. Disabled all plugins, and then, to make sure there was no conflict, renamed the plugins folder
2. Changed to a default WordPress theme
3. Added “define(‘CONCATENATE_SCRIPTS'”, false); to wp-config.php
4. Made sure all file permissions were correct
5. Deleted the wp-admin folder and wp-includes folder and uploaded fresh copies.
6. Checked the .htaccess file – it doesn’t show anything other than the standard WordPress mod_rewrite code.
7. I read on another thread that “Use Google Libraries” plugin would help, so I installed it. No change.
8. Disabled and re-enabled the visual editor.
9. Upgraded php to 5.4
10. Enabled Debug mode – it shows no errors

I’m open to any ideas, because I’m stumped.