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