Roy will at least be pleased to know he is no longer the only person to experience this. Perhaps with two of you involved we can identify the cause.
Pleaser try this possibility. (No guarantees, but Roy mentioned that it helped his issue). Replace the file admin_trees_manage.php in the kiwitrees root directory with the one in the attached zip folder. Then start a completely fresh installation. To do that just delete the file config.ini.php in your folder kiwitrees/data/. It is not necessary to delete the database, and you can re-use the same one.