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Well! I've solved the logout issue about DNN 4.8.2 version.

Go under admin\Authentication directory of your DNN installation and open the Logoff.ascx.vb file. Here you must modify the redirect sub in this way:

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         Private Sub Redirect()
            ' Redirect browser back to portal logout page
            Response.Redirect(RedirectURL, True)
        End Sub

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Then add the redirectURL property (copy and paste below)

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         Protected ReadOnly Property RedirectURL() As String
            Get
                Dim _RedirectURL As String = ""

                Dim setting As Object = UserModuleBase.GetSetting(PortalId, "Redirect_AfterLogout")

                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"))
                        ' 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
                    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 logout page
                    _RedirectURL = NavigateURL(CType(setting, Integer))
                End If
                'replace language parameter in querystring, to make sure that user will see page in correct language
                If UserId <> -1 Then
                    If User.Profile.PreferredLocale <> CultureInfo.CurrentCulture.Name Then
                        _RedirectURL = UrlUtils.replaceQSParam(_RedirectURL, "language", User.Profile.PreferredLocale)
                    End If
                End If

                'check for insecure account defaults
                Dim qsDelimiter As String = "?"
                If _RedirectURL.Contains("?") Then
                    qsDelimiter = "&"
                End If

                Return _RedirectURL

            End Get
        End Property

Now the logout redirect to logout page setted in the user setting or to home page if nothing is setted