Group users can't delete and manage cache/storage files. The permissions, being set 755, disallow group users to handle such files.

steps to reproduce

• install a fresh ezstudio 16.02 (with demo content)
• verify the permission settings in web/var/site/storage/images/_aliases

An alternative to setup less restrictive filesystem permissions is to document them as being like this and instructing site maintainers to ensure all cronjobs are executed by apache user.