I have updated to WP 5.8 and am using PHP 7.4 and am seeing various PHP warnings & errors relating to blocks and widgets.

I have already changed the definition of ABSPATH in my wp-config.php file from:
define('ABSPATH', dirname(__FILE__) . '/');
to
define('ABSPATH', __DIR__ . '/');

I am aware that WP 5.8 introduced new widget blocks but I have not started using that feature. I am assuming I do not have to install the classic widgets plugin for existing pages to continue working?

Below is a summary of the errors being logged. Any help will be much appreciated.

(E1) Fatal error: Uncaught Error: Class 'WP_Widget' not found in /home/xxx...xxx/wp-includes/widgets/class-wp-widget-block.php:17

(E2) Fatal error: require(): Failed opening required 'ABSPATHWPINC/blocks/archives.php' (include_path='.:/opt/cpanel/ea-php74/root/usr/share/pear') in /home/xxx...xxx/wp-includes/blocks/index.php on line 9

(E3) Fatal error: Uncaught Error: Call to undefined function get_the_block_template_html() in /home/xxx...xxx/wp-includes/template-canvas.php:12

I am also seeing warnings that might give a clue

(W1) Warning: require(ABSPATHWPINC/blocks/archives.php): failed to open stream: No such file or directory in /home/xxx...xxx/wp-includes/blocks/index.php on line 9

(W2) Warning: Use of undefined constant WPINC - assumed 'WPINC' (this will throw an Error in a future version of PHP) in /home/xxx...xxx/wp-includes/blocks/index.php on line 9

(W3) Warning: Use of undefined constant ABSPATH - assumed 'ABSPATH' (this will throw an Error in a future version of PHP) in /home/antipole/public_html/ovni-owners/wp-includes/blocks/index.php on line 9

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