Update. As it seems that my wordpress instance does not have the 'wp_' prefix on most of the database tables, it was 3 new lines in the updated /etc/wordpress/wp-config.php that caused the problem:
if (!isset($table_prefix)) {
$table_prefix = 'wp_';
}
The fix was to restore the wordpress database from backup and then add one line, setting $table_prefix to '' in my site's /etc/wordpress/config-default.php where the database passwords are.
Update. As it seems that my wordpress instance does not have the 'wp_' prefix on most of the database tables, it was 3 new lines in the updated /etc/wordpress/ wp-config. php that caused the problem:
if (!isset( $table_ prefix) ) {
$table_prefix = 'wp_';
}
The fix was to restore the wordpress database from backup and then add one line, setting $table_prefix to '' in my site's /etc/wordpress/ config- default. php where the database passwords are.
$table_prefix = '';
And then run the Automatic Updater normally: localhost/ wordpress/ wp-admin/ update- core.php
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