Asp Ajax Gridview Error The status code returned from the server was: 500

Hello everyone, I am working with a gridview that need to be implemented as a Excel Spreadsheet.  My gridview has many columns that I populate programmatically using the following code:

Dim ad As New SqlDataAdapter(mySql, myConnection)       

Dim ds As New DataSet()

        ad.Fill(ds)

        gvTimeUser.DataSource = ds

        gvTimeUser.DataBind()  'gvTime is my gridview

the first 5 gridview's columns should work as a cascade dropdown list. When I get the focus on the first column I have to change the text control into a dropdownlist and be able to chose a value. When I change that dropdownlist automatically I have to change the second column into a dropdownlist and retrieve values according the first column value(Dropdownlist cascade). This cascade is similar for the remaining three columns. The application is working fine but when my gridview retrieve more than 56 rows I got the error:  The status code returned from the server was: 500. I think is the timeout. I changed in the web.config but I am still getting the same error. Please help me to fix this problem.

I would like to add that mygridview is inside an asp:UpdatePanel (I am using Ajax)

Thanks