Kenneth: Hi Tux, Move the uploader control out of the ModalPopupExtender, does it work for your us clients? Does it got problem with the special file/size? Or randomly? What silverlight version your us client using? Regards, Ken Hi Ken, today we finally managed to solve the issue. As for the italian user, her problem was SilverLight (it wasn't installed on her PC). As for the two US users, they had to install SilverLight too, but it wasn't enough. We had to removed this HTML tag: <meta http-equiv="X-UA-Compatible" content="IE=8" /> from the MasterPage which includes the ContentPage where the ModalPopupExtender (with the uploader) is. It was somehow interfering with the AjaxUploader, because as soon as we removed it, the problem disappeared. Regards, Tux.