I tried logging into one of my websites just now, and got an empty page instead of the login fields. The URL also did not go to the usual destination, but to domainname/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F instead.
Time to fire up trusty Google…
And then I found the (simple) solution, from chowsir:
If you are getting the yoursite.com/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F error, this worked for me:
- Navigate via FTP to your installation. Check to see if the upgrade.php file is there.
- If not, download WordPress 3.4.2 from www.wordpress.org.
- Unzip the download and navigate to the wp-admin folder.
- Locate the upgrade.php file and FTP it to your installation /wp-admin folder
Somehow the in-app update process download was corrupted and missed the required upgrade.php which the redirect points to after the update.