On our DNN portal we have a custom authentication module which creates an authentication cookie. We need the cookie because we are trying to make a single sign-on solution for our DNN portal and our web application. But something overrides the cookie when the user gets logged in.

I assume that the cookie gets destroyed by something int he OnUserAuthenticated method. What does that method do other than redirection?

Both the DNN portal and the web application are already in the same IIS website.

They are separate applications inside the same IIS website.