System.InvalidOperationException was unhandled

These are my code:

Sub AttemptLogin(ByVal val As String)
        Dim MDBConn As OleDbConnection
        Dim MDBcmd As OleDbCommand
        Dim MDBReader As OleDbDataReader
        Dim ip_check As Integer
        Dim IP_Address As String

            Dim frmConUser As New frmConnectUser
            frmConUser.StartPosition = FormStartPosition.CenterParent
            If val = "REFUSE" Then
                frmConUser.lblAlert.Text = "**The user account is logged on**"
                client.GetStream.Dispose()
                client.Close()
            ElseIf val = "WRONG" Then
                frmConUser.lblAlert.Text = "**The user account is not exsit or the password is not match!**"
                client.GetStream.Dispose()
                client.Close()
            End If
            frmConUser.ShowDialog()
            If frmConUser.txtUserLogin.Text <> "" Then
                MDBConn = New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=Database.mdb;")
                MDBConn.Open()
                MDBcmd = New OleDbCommand("select * from serverlist order by id", MDBConn)
                MDBReader = MDBcmd.ExecuteReader

                While MDBReader.Read() And ip_check <> 1
                    Try
                        IP_Address = CStr(MDBReader(1))
                        client = New TcpClient(IP_Address, PORT_NUM)
                        client.GetStream.BeginRead(readBuffer, 0, READ_BUFFER_SIZE, AddressOf DoRead, Nothing)
                        ip_check = 1
                        MDBReader.Close()
                        MDBConn.Close()
                        SendData("CONNECT|" & frmConUser.txtUserLogin.Text & "|" & frmConUser.txtPassword.Text)
                        'frmConnectUser.Dispose()
                        'frmConnectUser.Close()
                        mnuConnect.Text = "Disconnect"
                        Exit While
                    Catch ex As Exception
                        ip_check = 2
                    End Try
                End While

                If ip_check = 2 Then
                    MsgBox("Server is not active.  Please start server and try again.", MsgBoxStyle.Exclamation, Me.Text)
                    Exit Sub
                End If
            ElseIf mnuConnect.Text = "Disconnect" Then
                SendData("DISCONNECT")
                MarkAsDisconnected()
                client.Close()
                mnuConnect.Text = "Connect"
            End If
    End Sub

I have got error msg for "frmConUser.ShowDialog()" when I run twice times:

System.InvalidOperationException was unhandled
  Message="復原作業的內容與套用至對應設定作業的內容不同。可能是因為內容已設定在執行緒上，但沒有還原 (復原)。"
  Source="mscorlib"
  StackTrace:
       ÃƒÂ¦Ã‚–¼ System.Threading.SynchronizationContextSwitcher.Undo()
       ÃƒÂ¦Ã‚–¼ System.Threading.ExecutionContextSwitcher.Undo()
       ÃƒÂ¦Ã‚–¼ System.Threading.ExecutionContext.runFinallyCode(Object userData, Boolean exceptionThrown)
       ÃƒÂ¦Ã‚–¼ System.Runtime.CompilerServices.RuntimeHelpers.ExecuteBackoutCodeHelper(Object backoutCode, Object userData, Boolean exceptionThrown)
       ÃƒÂ¦Ã‚–¼ System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       ÃƒÂ¦Ã‚–¼ System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
       ÃƒÂ¦Ã‚–¼ System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       ÃƒÂ¦Ã‚–¼ System.Net.ContextAwareResult.Complete(IntPtr userToken)
       ÃƒÂ¦Ã‚–¼ System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
       ÃƒÂ¦Ã‚–¼ System.Net.Sockets.BaseOverlappedAsyncResult.CompletionPortCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
       ÃƒÂ¦Ã‚–¼ System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)