Form authentication fails (AJAX via SSL)

All,

I'm running into a problem with my AJAX requests when using SSL. When SSL is disabled, it works correctly; with SSL enabled, the first AJAX request goes through, and each subsequent request fails. I would appreciate any help or hints to narrow it down.

Here are some relevant code snippets and a description of the problem scenario:

*****ajax.js:*****

function AjaxRequestObject() {
 this.Create = function() { 
  var ro;
  try {
   ro = new XMLHttpRequest();
  }
  catch (e) {
   ro = new ActiveXObject("Microsoft.XMLHTTP");
  }
  return ro;
 }
 this.SendRequest = function(action, url, data) {
     var isAsync = false;  // assume call will be synchronous, unless a success function is defined
  if (typeof this.SuccessFunction ==  ...