It seems like the umask settings fo php-fpm are not overwritten correctly, leading to the cache files being not group-writable (for group www, where the deploy user is a member). The umask settings should be overwritten by the file /etc/systemd/system/php7.3-fpm.service.d/override.conf (generated through our infrastructure playbooks).

To further investigate:

• Deploy to production and check files there, to check if this was a change in Twig (which we updated recently).
• Check if the systemd settings are overwritten correctly (e.g. systemd startup log)
• Check if this is a PHP 7.3 problem (we're already running PHP 7.3 on the test server)
• Update server to a kernel >= 4.7, to be able to check the umask settings.

I had another look and found the error: Ubuntu 14.04 does not use Systemd yet (it uses upstart) and in preparation for the Update to 18.04 I removed the upstart configuration (PR https://github.com/wmde/fundraising-infrastructure/pull/254), misreading the info at https://wiki.ubuntuusers.de/systemd/ (German site).

Conclusion: T214461: Setup VMs for test and production will very likely solve this problem.