This is a known bug, to fix add this code inside the On_Init event.
Response.AddHeader( "p3p", "CP=\"IDC DSP COR ADM DEVi TATi PSA PSD IVAi IVDi CONi HIS OUR IND CNT\"" );
I suggest that you put in on a master page or, as I do: I extend the System.Web.UI.Page and override the function:
//---------------------
// OnInit
//---------------------
protected override void OnInit ( EventArgs e )
{
// Header para resolver problema de sessão dentro de frames
Response.AddHeader( "p3p", "CP=\"IDC DSP COR ADM DEVi TATi PSA PSD IVAi IVDi CONi HIS OUR IND CNT\"" );
}
Then in my page I use this class as the base.
Hope it helps!