There has been a critical error on this website

For me also same problem.

Postie Configuration Test

There has been a critical error on this website. Please check your site admin email inbox for instructions.

Learn more about troubleshooting WordPress.

I’d like to assist but that isn’t nearly enough information. Do you have access to your web server logs that give more details?

Fatal error: Uncaught Error: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function “_return_false” not found or invalid function name
in /var/www/html/wp-includes/class-wp-hook.php on line 310

Call stack:

1. WP_Hook::apply_filters()
   wp-includes/plugin.php:205
2. apply_filters()
   wp-includes/comment.php:688
3. wp_allow_comment()
   wp-includes/comment.php:2278
4. wp_new_comment()
   wp-content/plugins/postie/postie-message.php:682
5. PostieMessage::save_post()
   wp-content/plugins/postie/postie-message.php:159
6. PostieMessage::process()
   wp-content/plugins/postie/postie.class.php:211
7. Postie::fetch_mail()
   wp-content/plugins/postie/postie.class.php:89
8. Postie::get_mail()
   wp-content/plugins/postie/postie-api.php:74
9. postie_get_mail()
   wp-content/plugins/postie/config_form.php:33
10. require_once()
    wp-content/plugins/postie/postie.php:302
11. PostieInit::postie_show_config_page()
    wp-includes/class-wp-hook.php:308
12. WP_Hook::apply_filters()
    wp-includes/class-wp-hook.php:332
13. WP_Hook::do_action()
    wp-includes/plugin.php:517
14. do_action()
    wp-admin/admin.php:259

Thanks. This is due to running PHP 8. I am working on making Postie PHP 8 compatible.

In the meantime downgrading to 7.4 would solve the issue.

The latest version of Postie (1.9.64) should fix this issue.