Parallel transactions and entity framework

I've a web service used to append data from source A (company ERP) to source B (sql production database). Web Service can be invoked n times.
Sometimes an exception occured: System.InvalidOperationException: SqlConnection not support parallel transaction and the specific code area where I suppose is located the problem is this sub:

 Friend Sub Aggiungi_AnaCarichi()
        Dim NuovoPianoCarico As New AnaCarichi
        With NuovoPianoCarico
            .NumPianoCarico = _NumPianoCarico
            .DataCarico = _DataCarico
            .DataElaborazione = Date.Now
            .Descrizione = _Descrizione
            .IDStato_PCarico = "0"
        End With
        dbLineProdContext.AddToAnaCarichi(NuovoPianoCarico)
        Try
            dbLineProdContext.SaveChanges()
            ' Salvo il valore della colonna identity IDPianoCarico, mi servirà quando dovrò salvare
            ' i dati in Carichi_Padri, Carichi_PadriVarianti e Carichi_Figli.
            _IDPianoCarico = NuovoPianoCarico.IDPianoCarico
        Catch ex2 As OptimisticConcurrencyException
            dbLineProdContext.Refresh(Objects.RefreshMode.ClientWins, dbLineProdContext.AnaCarichi)
            dbLineProdContext.SaveChanges()
            _IDPianoCarico = NuovoPianoCarico.IDPianoCarico
        Catch ex3 As UpdateException
            pvclErrori.Codice = 1
            pvclErrori.Descrizione &= "Salvataggio nuovo p.d.c. in AnaCarichi: " & ex3.Message & vbCrLf
        End Try
    End Sub

I use entity framework. AnaCarichi is an entity of the entity model dbLineProdContext.
What can I do to solve the problem?