[Erel]

You don't need to initialize filestream object each time you read the data.
Instead you should initialize once after creating the connection.

Maybe something like this will solve your problem:
Instead of:

Sub Timer2_Tick
    ErrorLabel(connectionbreak)
    If ComboBox2.SelectedIndex = 1 Then
        If client.DataAvailable=true Then
            Image4.LoadPicture("IconDaten1.bmp")
            'REMOVE THIS LINE:filestream.New1(client.GetStream,false)
            GPS_Server.GPSStream(filestream.ReadString)
        Else
            Image4.LoadPicture("IconDaten2.bmp")
        End If
    End If
    Return
    
    connectionbreak:
    ..
    client.close
End Sub

Discard the data if ComboBox2.SelectedIndex doesn't equal 1:

Sub Timer2_Tick
    ErrorLabel(connectionbreak)
    If ComboBox2.SelectedIndex = 1 Then
        If client.DataAvailable=true Then
            Image4.LoadPicture("IconDaten1.bmp")
            GPS_Server.GPSStream(filestream.ReadString)
        Else
            Image4.LoadPicture("IconDaten2.bmp")
        End If
    Else If client.DataAvailable Then
       fileStream.ReadString
    End If
    Return
    
    connectionbreak:
    ..
    client.close
End Sub