WordPress downgrade / changing version
Problem: when you update / downgrade wordpress, you have following error:
This is usually due to inconsistent file permissions.: xmlrpc.php, wp-cron.php, wp-config-sample.php, wp-login.php, wp-settings.php, wp-mail.php, wp-links-opml.php, wp-load.php, wp-trackback.php
Solution: if you can access your host via ssh, cd the wordpress installation folder and run following command:
sudo chown -R www-data:www-data .
Attention! Make sure that user which runs wordpress is www-data.