an unknown error has occurred while processing the request on the server. the status code returned from the server was 500

What code you have written? Can you please share.

Hi Pawan thanks for response. Here is the sample code.

I have a button and an gridview inside update panel.

<asp:UpdatePanel ID="sample" runat="server" >
 <contentTemplate>

       <button type="button" runat="server" onclick="javascript: LoadingModal();" onserverclick="GenerateReport" />

     <asp:GridView ID="gridViewRpt" runat="server">
     </asp:GridView>


 </ contentTemplate>
</asp:UpdatePanel>

CODE BEHIND:

Protected Sub GenerateReport(sender as object, e as EventArgs)
........
gridViewRpt.DataSource = cmd.ExecuteReader()
gridViewRpt.DataBind()
End Sub

when i click the button on the first time, there is no error, but when i click it again this error is occured.

Hi Ryan Chong, how can I prevent this kind of error?

how can I prevent this kind of error?

try to verify what's the exact error generated from server end...

for example, if you can't just-in-time debugging... try do a try catch in Sub GenerateReport and send that error into a custom log file for debugging.

Just-In-Time Debugging in Visual Studio
https://msdn.microsoft.com/en-us/library/5hs4b7a6.aspx?f=255&MSPPError=-2147217396

for the custom log, we usually use log4net library which you can get from NuGet Manager.

log4net
https://www.nuget.org/packages/log4net/