After updating your WordPress core to a new version, a message may appear when you try to log in to the wp-admin – “Database update required“. The message prompts you to update the WordPress database, but when you click “Update WordPress Database” it returns “No update required, your WordPress database is already up to date“. Then, if you try to log in to your wp-admin again, the same “Database update required” message appears and you cannot continue to your admin panel.
To resolve the problem, find the file named object-cache.php located in the wp-content folder of your WordPress installation, and rename or delete it. This will allow you to access the wp-admin properly again.
Deleting or renaming the file will disable the Memcached caching functionality of any caching plugins you are using on your website.