The problem is not reproducible anymore since Microsoft no longer supports Internet Explorer where the reported issue can be seen only.

Temporary workaround is to correct the text in the ProgressArea's OnClientProgressBarUpdating event handler:

function OnClientProgressBarUpdatingHandler(sender, args) {
    if ($telerik.isIE9 || $telerik.isIE8 || $telerik.isIE7) {
        var upload = $find("<%= RadAsyncUpload1.ClientID %>");
        var cancelButtonText = upload.get_localization().Cancel;
        var text = args.get_progressData().CurrentOperationText;

        if (text.endsWith(cancelButtonText)) {
            var updatedText = text.slice(0, text.lastIndexOf(cancelButtonText));
            args.get_progressData().CurrentOperationText = updatedText;
        }
    }
}