“There has been a critical error on your website. Learn more about debugging in WordPress.” after updating to WordPress 5.7 (All in one WP security and firewall) [Solved]

Investigating the error log reveals that the problem has to do with All in one WP security and firewall plugin:

Got error 'PHP message: PHP Fatal error:  Cannot redeclare retrieve_password() (previously declared in /www/wp-includes/user.php:2671) in /www/wp-content/plugins/all-in-one-wp-security-and-firewall/other-includes/wp-security-rename-login-feature.php on line 358'

The problem has been solved in All in one WP security and firewall plugin version 4.4.8, so it means your WordPress site was updated to 5.7 prior to updating the plugins.

To solve it, you need to update All in one WP security and firewall but not being able to login to your site you will need to solve this using FTP.

Using FTP, rename the folder wp-content/plugins/all-in-one-wp-security-and-firewall to something else. Now you can log into your site and reinstall All in one WP security and firewall. Make sure you get at least version 4.4.8.