xhr.status == 0 errors in ajax calls with 10% of IE8 users in ujs 2.1.2

[holli]

We upgraded to newest ujs from version 1.0.19 and started getting complaints from IE8 users that our ajax buttons won't work. From the logs we were calculating that anything between 5%-15% of IE8 users had some problems. Problem was that we were not able to confirm this with our IE8 machines so we couldn't verify what's the problem.

We tested running totally same setup on the production, only ujs javascript was changed. With new ujs we had thousands of xhr.status == 0 errors in our analytics (and also some complaints from our users)...

I tried to check what would cause the problem by diffing ujs versions but I don't have any ideas:
http://diffchecker.com/X4ZlzlED

All the problematic ajax buttons had data-disable-with info but I don't think thats the problem. Some other ajax functions might have worked allright but it's hard to confirm this from normal users. All the advanced users use some decent browsers anyway :) E.g. draganddrop migth been functional. Go figure.

This is how we log the errors. Both to google analytics and to our server.

$jq(document).ajaxError(function(e, xhr, settings) {
    _gaq.push(['_trackEvent', 'AjaxError', "xhr.status_"+xhr.status+"_first", settings.url]);
    params = { category: 'js_error.ajaxError', 'xhr.status': xhr.status }
    $jq.post("/common/log_request", params);
});

Only common thing in user_agent infos was MSIE 8.0. Url, button, operating system etc differed.

So basically the ajax call was newer sent to our server. It resulted in ajaxError right away with xhr.status = 0.