Not sure if this is a bug or a local problem.
A bit of both ๐ The MYSQL function mentioned in the bug report (“sql_mode=only-full-group-by”) became “on by default” from MySQL 5.7.5, which kiwitrees is not fully compatible with. But I am squashing the few minor bugs preventing that compatibility as I find them.
Please replace the file kiwitrees/includes/functions/functions_db.php with the one in the attached zip folder and let me know if it cures this issue.