AJAX login not working only for some users

login doesn't work for some users no matter what browser they try

[test1]

Hey everyone. I'm having an issue with my login script and I can't seem to track down the problem. It seems to be completely browser independent and it is only effecting a few users. Basically when they enter in their username and password it always seems to give them an error saying the login is incorrect. I've included the code that I'm using to make some of the AJAX requests. Is there anything in here that someone could see that might cause some users a problem?

// send command to php
function sendReq(url,responder,data){
onComplete = responder;
 // branch for native XMLHttpRequest object
 if (window.XMLHttpRequest) {
  req = new XMLHttpRequest();
  req.onreadystatechange = processReqChange;
  req.open('POST', url, true);
  req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  req.send(data);
 // branch for IE/Windows ActiveX version
 } else if (window.ActiveXObject) {
  req = new ActiveXObject('Microsoft.XMLHTTP')
  if (req) {
   req.onreadystatechange = processReqChange;
   req.open('POST', url, true);
   req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
   req.send(data);
  }
 }
}

[Demaestro]

Are you sure that this isn't user error and they are in fact entering their login information wrong?