Hi,
I succeed to fix this by changing user, group and chdir:
/etc/php/7.4/fpm/pool.d/synapse.conf
[synapse]
user = matrix-synapse
group = matrix-synapse
chdir = /var/www/synapse
listen = /var/run/php/php7.4-fpm-synapse.sock
listen.owner = www-data
listen.group = www-data
pm = ondemand
pm.max_children = 16
pm.max_requests = 500
request_terminate_timeout = 1d
pm.process_idle_timeout = 10s
; Additional php.ini defines, specific to this pool of workers.
php_admin_value[upload_max_filesize] = 100M
php_admin_value[post_max_size] = 100M
And restart:
systemctl restart php7.4-fpm