Kiwitrees 3.3.5 (with the same database):
https://www.motta.pt/newkw
A new 3.3.5 gives me the same error.
I am pleased you have upgraded to 3.3.5. I cannot provide support for a version as old as 2.0.4 I’m afraid.
Your link shows this message:

It says that you need to upgrade PHP. I recommend version 7.2.
I think the file functions_db.php never exists in kiwitrees 2.04
Kiwitrees has always, and still does include the file /includes/functions/functions_db.php