Sys.WebForms.PageRequestManagerServerErrorException:

Problem: - Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500

Solution: -

In the @Page tag set 

ValidateRequest="false"

When does it occur?

This normally occurs when there is 

1) a multiline textbox in the page

2) an update panel in the page inside which the multiline textbox resides

121 unique visitors

The GridView 'gvManageProducts' fired event RowDeleting which wasn't handled.

To solve this exception do these:-

protected void gvManageProducts_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
//required -do not remove
}

and also write this:-

protected void gvManageProducts_SelectedIndexChanged(object sender, EventArgs e)
{
//required -do not remove
}

The GridView 'gdABCD' fired event PageIndexChanging which wasn't handled.

1) If we change DataGrid to a GridView so inside any Event we change
DataGridPageChangedEventArgs to GridViewPageEventArgs.

2) If you set AllowPaging="true" or AllowSorting="true" on a GridView control without using a DataSourceControl DataSource (i.e. SqlDataSource, ObjectDataSource), you will run into the following errors: -

When changing the page on the GridView control: -

The GridView 'GridViewID' fired event PageIndexChanging which wasn't handled.
When clicking a column name to sort the column on the GridView control: -

The GridView 'GridViewID' fired event Sorting which wasn't handled.

As a result of not setting the DataSourceID property of the GridView to a DataSourceControl DataSource, you have to add event handlers for sorting and paging.