In the file kiwitrees/includes/functions/functions_db.php change line 1669 (or close) from
return WT_I18N::number(($size / 1024000), 1) . ' MB';
return $size / 1024000 . ' MB';
Let me see what that looks like, then it should be possible to see where the error is coming from.
Also, have you checked to see if it is the same in all languages?