The version of kiwitrees you are using (3.2.3) is 2 years old and no longer maintained. It is now not compatible with the version of PHP running on your web server.
You will either need to update kiwitrees (https://kiwitrees.net/services/downloads/) or go back to an older version of PHP (version 7.0 or earlier). But that is NOT recommended. Those versions of PHP are also no longer maintained, so might have security risks.