SAP B1 Service Layer, Error by posting SalesOrder PostAsync C#.NET, possible bug

Good morning, dear SAP Developer!

I'm using service layer for posting sales order. I use HttpClient, but it doen't work. It works fine with HttpWebRequest, but not with HttpClient. I get connection with HttpClient, I can read existing data from DB with HttpClient.GetAsync method, but it's not possible to create new sales order with PostAsync. At the same time I can do it without any problems with HttpWebRequest, that becomes connection's cookies from the HttpClient, which was conneted earlier. Please verify this case. Is it a bug? Or does it not work only in my development environment?

Best regards,

Alexander