This bug was also in 4.5.1. Allthough i'm still on 4.8.1 and i haven't test it yet, i guess that the solution from these times will still work.
In Directory admin\security change the procedure RedirectURL in the File signin.ascx.vb:
Protected ReadOnly Property RedirectURL() As String
Get
Dim _RedirectURL As String = ""
Dim setting As Object = UserModuleBase.GetSetting(PortalId, "Redirect_AfterLogin")
If CType(setting, Integer) = Null.NullInteger Then
If Not Request.QueryString("returnurl") Is Nothing Then
' return to the url passed to signin
_RedirectURL = HttpUtility.UrlDecode(Request.QueryString("returnurl"))
ElseIf PortalSettings.LoginTabId <> -1 And PortalSettings.HomeTabId <> -1 Then
' redirect to portal home page specified
_RedirectURL = NavigateURL(PortalSettings.HomeTabId)
Else
' redirect to current page
_RedirectURL = NavigateURL(Me.TabId)
End If
Else ' redirect to after login page
_RedirectURL = NavigateURL(CType(setting,Integer))
End If
Return _RedirectURL
End Get
End Property
That always worked in every version i tested having this bug.
Kai
PS: Do not forget to make a backup ;-)