Sorry to dig up an old topic, but I’m having the same behavior, with nearly the same error message:
2023/08/14 17:31:11 [error] 168493#168493: *114996 FastCGI sent in stderr: "PHP message: PHP Warning: Undefined array key "id" in /var/www/wordpress/wp-content/plugins/wp-fail2ban/vendor/freemius/wordpress-sdk/templates/connect/permission.php on line 29PHP message: PHP Warning: Undefined array key "id" in /var/www/wordpress/wp-content/plugins/wp-fail2ban/vendor/freemius/wordpress-sdk/templates/connect/permission.php on line 29PHP message: PHP Warning: Undefined array key "id" in /var/www/wordpress/wp-content/plugins/wp-fail2ban/vendor/freemius/wordpress-sdk/templates/connect/permission.php on line 30" while reading upstream, client: 0.0.0.0, server: example.com, request: "POST /wp-admin/admin.php?page=wp-fail2ban_addon_blocklist HTTP/2.0", upstream: "fastcgi://unix:/run/php/php8.1-fpm.sock:", host: "www.example.com", referrer: "https://www.example.com/wp-admin/admin.php?page=wp-fail2ban_addon_blocklist
WP-fail2ban plugin version is 5.2.0, PHP 8.1, WordPress 6.3 on ubuntu server.
Any idea what happened?