The attached zip file can be used to fix it. Replace the file kiwitrees/autocomplete.php on your server with the version in this zip folder.
Just a reminder, “geonames” is a very large external gazetteer of place names. It will be slower finding places there than when simply returning a place you already have in your kiwitrees database (the latter is always checked before looking at geonames). So expect a at least a few seconds delay for a response. Speed will be improved by typing as you type more characters.