Hi, just a fellow user here, but there seems to be a very similar issue
here that is in french.
From what I understand they copied the missing firefly-iii.conf file from the php 7.4 folder to php 7.3 one to solve it.
cp /etc/php/7.4/fpm/pool.d/firefly-iii.conf /etc/php/7.3/fpm/pool.d/