Hi MarjaR,
Is your site online? If so, can you send me the uploader page url and show me the full steps to reproduce this issue? I will check it and get back to you as soon as possible.
Is your site using window authentication?
Is your site on the load balancing, web farm, cloud environment?
Can you reproduce this issue with the example below on your end?
<%@ Page Language="C#" %>
<%@ Register TagPrefix="CuteWebUI" Namespace="CuteWebUI" Assembly="CuteWebUI.AjaxUploader" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>example</title>
</head>
<body>
<form id="form1" runat="server">
<CuteWebUI:UploadAttachments ID="uploader1" runat="server" UploadType="iframe">
</CuteWebUI:UploadAttachments>
</form>
</body>
</html>
Regards,
Ken