@mike175de actually post OS upgrade php-fpm is not listening on port 9999 so like for Debian 10 to 11 iRedmail upgrade instructions, it's necessary to perform a:
cp /etc/php/7.4/fpm/pool.d/www.conf /etc/php/8.2/fpm/pool.d/www.conf
service php8.2-fpm restartotherwise nginx thrown an http 503 while attempting to access Roundcube