AJAX :: AsyncFileUpload "unknown Server Error"

Now, i want use 4 AsyncFileUpload and i added those to page. But three of them in unvisible div tag like

<div id="div3" runat="server">
<cc:AsyncFileUpload ID="afu2" runat="server" UploadingBackColor="#CCFFFF" ThrobberID="myThrobber2"
Width="300px" />
<asp:Label runat="server" ID="myThrobber2" Style="display: none;"><img align="absmiddle" alt="" src="uploading.gif" /></asp:Label>
</div>

for example when user press add button i do div3.visible=true and afu2 is visible

then press afu2's upload button it gives error server response error:"unknown server error"

i use updatepanel in my form

aspx code

[Code]....