The first thing to do is to find out which line is giving the error! Put a try...catch block around your method:
protected void SAVE_bt_Click(object sender, EventArgs e)
{
try
{
if (Hidden_txt.Text == Convert.ToString(1))
{
...
}
else
{
...
}
}
catch (Exception ex)
{
}
}
When you have that info, you can start to look at what might be the problem.