I upgraded Debian Buster to Bullseye carefully following the procedure. Part of the procedure includes removing old or no longer used packages, which I think deleted something that was actually needed.
Not to worry, I managed to fix it. I think php-fpm was missing/deleted. After running ‘apt -y install php-fpm’ it started working again.
Oddly, I received these messages installing php-fpm:
NOTICE: Not enabling PHP 7.4 FPM by default.
NOTICE: To enable PHP 7.4 FPM in Apache2 do:
NOTICE: a2enmod proxy_fcgi setenvif
NOTICE: a2enconf php7.4-fpm
NOTICE: You are seeing this message because you have apache2 package installed.
Created symlink /etc/systemd/system/multi-user.target.wants/php7.4-fpm.service -> /lib/systemd/system/php7.4-fpm.service.
Setting up php-fpm (2:7.4+76) …
Processing triggers for man-db (2.9.4-2) …
Processing triggers for php7.4-fpm (7.4.21-1+deb11u1) …
NOTICE: Not enabling PHP 7.4 FPM by default.
NOTICE: To enable PHP 7.4 FPM in Apache2 do:
NOTICE: a2enmod proxy_fcgi setenvif
NOTICE: a2enconf php7.4-fpm
NOTICE: You are seeing this message because you have apache2 package installed.
Yet, despite not running the enabling or configuration commands listed, it still worked.