I tried to report this as a bug but i couldn't work out how to create an item in te gemini web site.  The issue i found is with the url created when someone clicks through to register from the login page.  I found it was not passing on the returnurl correctly if you got sent to the login page form a request to a page where you need to logon

In login.ascx.vb you need to change

.

        Private Sub cmdRegister_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cmdRegister.Click

            If PortalSettings.UserRegistration <> PortalRegistrationType.NoRegistration Then
                Dim ReturnUrl As String = HttpContext.Current.Request.RawUrl
                If ReturnUrl.IndexOf("?returnurl=") <> -1 Then
                    ReturnUrl = ReturnUrl.Substring(0,ReturnUrl.IndexOf("?returnurl=") )
                End If
                ReturnUrl = HttpUtility.UrlEncode(ReturnUrl)

to

        Private Sub cmdRegister_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cmdRegister.Click

            If PortalSettings.UserRegistration <> PortalRegistrationType.NoRegistration Then
                Dim ReturnUrl As String = HttpContext.Current.Request.RawUrl
                If ReturnUrl.IndexOf("?returnurl=") <> -1 Then
                    ReturnUrl = ReturnUrl.Substring(ReturnUrl.IndexOf("?returnurl=") + 11)
                End If
                ReturnUrl = HttpUtility.UrlEncode(ReturnUrl)