Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack

You will get an error, but the code block below will trap it and you can get on with your life.

Try this:

using System.Threading.ThreadAbortException;

catch(ThreadAbortException ex)
{
    throw;
}

That message you're seeing really isn't an error you see at runtime - typically, its seen in the debugger in the middle of the catch portion of a try-catch-finally block. Are you stepping through the code in the debugger, or are you actually getting the error as the application runs outside the debugger?

If you're trying to determine what error is really occurring, I'd recommend putting a full try-catch block around the Response.Redirect call and inspect the exception that fires at that point, eg

Try
   Response.Redirect(http://www.targetsite.com/index.php")
Catch ex as Exception 
   ' add your handling code here, using ex as the Exception placeholder variable
End Try

Good luck!