FormsAuthenticationTicket Error

I get below error while trying to open page, but error disappears once i clear my browser data. The error always occur when am already login and tries to lunch another application.

if (authCookie != null)
Line 27:             {Line 28:                 FormsAuthenticationTicket authTicket = FormsAuthentication.Decrypt(authCookie.Value);Line 29:                 if (authTicket != null && !authTicket.Expired)
Line 30:                 {