ISPConfig 3.2 is not yet compatible with PHP 8.x. It may happen then when installing PHP8 on hosts running ISPConfig you’ll get a blank page when opening the management site

The ISPConfig admin website doesn’t use PHP-FPM but PHP-FCGI. It’s configured in /etc/apache2/sites-available/ispconfig.vhost in this line:

FCGIWrapper /var/www/php-fcgi-scripts/ispconfig/.php-fcgi-starter .php

The script above calls

exec /usr/bin/php-cgi

which is handled by alternatives, in Debian/Ubuntu. So in order to restore ISPConfig backend just run:

update-alternatives --configure php-cgi

and choose the latest available PHP 7.x version, hopefully 7.4.