Hi,

I just got this error today. It came after an update of php7.0-fpm and a replacement of /etc/php/7.0/fpm/pool.d/www.conf
I just had to restore the old file :

cp /etc/php/7.0/fpm/pool.d/www.conf.dpkg-old /etc/php/7.0/fpm/pool.d/www.conf

Hope it will work for you.