I found there was no upgrade.php file in the wp-admin.
I uploaded one and problem is now cured.
OK but seems a waste of your time as it is the same error!
Having same problem on 2 sites.
http://www.domain.com/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F displays and returns a 404 error.
The sites are still up and working but can’t access the wp-admin area.
How do I revert to 3.5.1 please?
Thanks
