Request["InnerErr"]
is null. you can't call ToString method of null object. it will throw NullReferenceException
you can check null and then call to string method like below
if(Request["Err"]!=null)
Label_ErrorFrom.Text = Request["Err"].ToString();
if(Request["InnerErr"]!=null)
Label_ErrorMessage.Text = Request["InnerErr"].ToString();
OR
protected void Page_Load(object sender, EventArgs e)
{
Label_ErrorFrom.Text = Request["Err"] as string;
Label_ErrorMessage.Text = Request["InnerErr"] as string;
}