UnblockUI call in ajax callback not working (IE 8)

I have the following code that I run on each page:

$(document).ready(function () {
    $(document).ajaxStart($.blockUI);
    $(document).ajaxStop($.unblockUI);
    $("form").submit(function() {
        if ($(this).valid() == true) {
            $.blockUI();
        }
    });
});

This code allows me to present a “loading ...” message during AJAX calls and form messages. In general, this works fine. However, I have a scenario in which I need to make an AJAX call to validate a piece of data before creating a form message as follows:

$.ajax({
    url: '/Item/VerifyFolder/',
    type: 'POST',
    dataType: 'json',
    data: {
        folderName: folderName
    },
    success: function (data, textStatus, jqXHR) {
        $.unblockUI(); 
        if (data.folderItemExists == true)
        {
            $("form").submit();
        }
        else
        {
            if (confirm("The folder specified for this item does not exist.  Do you want to create it?") == true)
                {
                    // ...
                    $("form").submit();
                }
                else
                {
                    return false;
                }
            }
        },
        error: function (jqXHR, textStatus, errorThrown) {
            DisplayError(textStatus);
        }
    });

In my AJAX callback, I start with the call $.unblockUI(). However, this does not seem to have any effect. As a result, the message “Loading ...” is still displayed when a warning appears.

I have seen various posts on the Internet about problems with IE 8 and the unblockUI method (garbled HTML, requiring a meta tag, etc.), but I have yet to find a solution that works.

. !