Problem: You update/upgrade WordPress to a new version. All looks great until you log into the admin area. You get a error page and the page redirect to a url that includes “/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F”.
Fix: After clearing cache and the usual errors, there is another possible culprit. The db_version in the database and in the config files do not match. To fix this, open the database in PhpMyAdmin, in the wp_options table, look at the db_version field. Now FTP to the site, and look at the wp_db_version variable in the file /wp-includes/version.php files. If they do not match, this is your likely problem. In most cases, the database needs to be edited to match the version in the file. Change the version number in the database, then open the WP admin in a new browser and try to login again. Hopefully this solves your problem.