I had the same problem. Here’s how I fixed it:
I clicked tools => upgrage => re-install Automatically. I got an error message about an unexpected “&” in a file called “class-wp-upgrader.php”. I then went back to the admin panel and clicked tools => upgrage => donwload 2.8.4. I found the new class-wp-upgrader.php file and ftp’d it to the server. I then went back and clicked tools => upgrage => re-install Automatically. When it re-installed it solved my problem. Hallelujah!
