Ganz einfache Lösung eigentlich.

chatbox.js
Code:
function meldung ()
{
	if(xmlHttp2.readyState == 4) {
		response = xmlHttp2.responseText;
		if (response.length > 20) window.alert(response);
		else {
			document.chatbox.message.value = '';
			document.chatbox.message.focus();
		}
		loadData();
	}
}