Jquery ajax仅对某些用户失败（ajax响应0）

Question

I have an ajax-based login system. The user types in their username/password and submits. This is fired asynchronously to a php script that validates etc and then logs the user in or passes back an error message in json.

The issue is that the ajax call is returning an error for a few users but not most. Weirder, when I try to debug by logging in as those users, it works fine using my own system. It seems linked to their specific setup somehow.

To debug I have the jquery ajax error handler email me the error data when logging in fails (along with some browser info). But the ajax errors I'm seeing are 'undefined'.

I'm at a loss as to how to proceed fixing this or further debugging it. I've proven that this isn't linked to a specific version of a browser, or a same origin issue. Any insight into this would be appreciated. Here's my code:

UPDATE I've managed to narrow the problem down to an ajax status response of 0. Some browsers are just stopping the process before it sends, but it's not consistent, leading me to believe it is some sort of plugin or extension that's causing the issue. But it's none of the obvious culprits - noscript, adblocker, etc.

$('#loginMain').on('submit', '#loginForm, #resetForm', function(e){
    e.preventDefault(); //don't submit the actual form
    $('#loginExtra').addClass('ajaxloader-circle'); //loading spinner
    var type = $(this).attr('id');
    var submitType = type.replace('Form', '');
    var uploadData = new FormData($("#" + submitType + "Form")[0]);  //get the form data          
    uploadData.append('token', '<?php echo $_SESSION['token']; ?>'); //prevent csrf 

    jQuery.ajax({
        url: 'https://www.example.com/ajaxfiles/loginProcess.php',
        data: uploadData,
        cache: false,
        contentType: false,
        processData: false,
        type: 'POST',
        dataType: 'json',
        success: function(data) { //on success, reload or present error message
            $('#loginExtra').removeClass('ajaxloader-circle');
            if (data['state'] == false) { //show error data if it failed
                $('#loginFeedback').html('<div style="color:red; text-align: center;">' + data['msg'] + '</div>'); //output any error messages
                $('#loginProcessing').html('');
            } else {
                $('#signupDynamic').html('Success! Reloading...');
                $('#signupFeedback').html('');
                $('#signupProcessing').html('');
                if (submitType == 'login') { //reload on successful login
                    var url = window.location.pathname;
                    var page = url.substr(url.lastIndexOf('/') + 1);
                    if (page != 'login') {
                        location.reload();
                    } else { //logging in on the login page
                        window.location.href = "https://www.example.com"; //redirect to home
                    }
                } else { //output on succesful reset
                    $('#loginFeedback').html('<div style="color:green; text-align: center;">' + data['msg'] + '</div>'); //output any error messages
                    $('#resetForm').html('');
                }
            }
        },
        error: function(xhr, textStatus, errorThrown){ //on fail, email me what went wrong
            var browser = '<?php echo $_SERVER['HTTP_USER_AGENT']; ?>'; 
            var errorText = textStatus + '|' + xhr.responseText + '|' + xhr.statusText +  '|' + xhr.readyState +  '|' + errorThrown + '|' + browser;
            $.post("https://www.example.com/ajaxfiles/errorTracker.php", {data: errorText, token: '<?php echo $_SESSION['token']; ?>'}, function(data) {
                window.location.href = "https://www.example.com/login.php";
            }, "text");
        }
    });

});

Answer 1

I was able to confirm that this WAS in fact caused by a cross-origin issue, which I had ruled out.

The user was somehow navigating to https://example.com (a redirect to www that should have been firing wasn't) which caused issues when the ajax call was sent to https://www.example.com.

Thanks for all the answers. They helped me narrow this down finally.