Looks like you might have wrong permissions on the kiwitrees /data/ folder. The system needs to be able to write to that folder, and it’s sub-folders, especially /data/cache.
From FAQs https://kiwitrees.net/faqs/introduction/security/
If your server runs PHP in CGI mode, then change the permission of the /data directory to 700 instead of 777. This will block access to the httpd process, while still allowing access to PHP scripts.