hi jon,

whenever I use a popup on a child portal, I get logged out (cookie gone).
Is there some way to prevent this?

Sebastian,

In my situation, the user in child portal will get logout after they got redirected back to the site by using Response.Redirect(http://MyServer/ChildPortal, True). It doesn't have this problem in parent portal, only this child portal.

I have installed a new DNN 4.3.7 with a child portal on another server, and it also doesn't have this problem.

Do you have any idea what might cause this problem?

Thanks a lot! 

I've seen this before with a response.redirect.
I think when you change it to
    Response.Redirect(NavigateURL(), True)
or
    Response.Redirect(EditUrl(ControlKey:="SomeControlKey"), True)
it will work.