Simple Ajax Code That Used To Work No Longer Works - ajax

Question

I have a custom webserver, and some content that has been working for probably 18 months or more. The code has not been touched in all that time. But, I now find one feature, which depends on Ajax, is no longer working. The problem is on the browser side. Sadly, the Ajax code was found using Google, and even when I customized it, I didn't really understand it, but it has worked until some time in the last 6 months or so. Now that it's no longer working, I'm kinda lost....
What the code does is pretty simple. My HTML form is a tab control. One of the tabs is used for displaying error messages. The Ajax code sends a GET request for "\ATCStatus.txt" roughly once per second. The server responds with:
HTTP/1.1
Server: arduino
Content-Type: text/plain
#ATCErrorFlag#
where #ATCErrorFlag# gets replaced with wither "true" or "false". If "false" is returned, nothing happens. If "true" is returned, then the Ajax code sends a request for "\ATCErrorPage.htm", which switches to the error tab, and displays the error message.
The server receives the requests, processes them, and sends the correct response, but Ajax code acts like no response was ever received, and I can't figure out why.
Anyone here know enough Ajax to have a clue?
Here is the Ajax code:
setInterval(PollATCError, 1000);
function PollATCError()
{
var xhr;
if (window.XMLHttpRequest)
xhr = new XMLHttpRequest();
else if (window.ActiveXObject)
xhr = new ActiveXObject("Msxml2.XMLHTTP");
else
throw new Error("Ajax is not supported by your browser");
xhr.onreadystatechange = function()
{
if (xhr.readyState == 4 && xhr.status == 200)
{
clearTimeout(xhrTimeout);
var response = xhr.responseText;
if (response == "false") {
// Do nothing
} else if (response == "true") {
// else, throw up Error Page
window.location.replace('ATCErrorPage.htm');
//document.getElementById("tab4event").click();
}
}
}
xhr.open('GET', 'ATCStatus.txt', true);
xhr.send(null);
var xhrTimeout = setTimeout("ajaxTimeout();", 900);
function ajaxTimeout()
{
xhr.abort();
// Note that at this point you could try to send a notification to the
// server that things failed, using the same xhr object.
}
}
What seems especially odd is that I don't ever seem to get any state change notifications. If I make the first line of the state change handler an alert(), the alert never comes up. The request does go to the server, the correct response is sent back to the client, but seems to never be seen by the XMLHttpRequest object. Even the 900mSec timeout never happens.
This used to work very nicely! What has gone wrong?? How can I get to the bottom of this?
Regards,
Ray L.