Case closed.  Problem I had was that I redirected to another domain than the one used by the portal itself, this required http:// to be included in the provided '?returnurl='

This has been prohibited in 5.0 by the following code.

                    If Not Request.QueryString("returnurl") Is Nothing Then

                        ' return to the url passed to signin

                        _RedirectURL = HttpUtility.UrlDecode(Request.QueryString("returnurl"))

                        ' redirect url should never contain a protocol ( if it does, it is likely a cross-site request forgery attempt )

                        '//If _RedirectURL.Contains("://") Then

                        '//    _RedirectURL = ""

                        '//End If

                    End If

a quick fix is as follows, however make sure to make this somewhat more advanced to prevent som phishing attacks.

Cheers