Edit: Everything below is pretty much superseded by a new post. php-fpm: /var/lib/php/session was owned by root:apache, so php-fpm couldn’t create session files. Fix was chgrp nginx /var/lib/php/session. For some reason, after doing  yum update, /var/lib/php/session was changed back to root:apache, but that was only noticed on one system so far, haven’t observed it on others, but […]