"There is already an open DataReader associated with this Command which must be closed first."

This can happen if you execute a query while iterating over the results from another query. I ran into this issue when I called another async call while I was executing one result.

public async Task<IActionResult> GetTicketsInventory(int customerID, int accountID)
        {
            var service_response = await this.ticketsService.GetTicketsInventory(customerID, accountID);
            List<TicketsInventoryCO> items = service_response.Response.ToConvert();

               // this is my second call where i am trying to get results based on some condition
               // to get customer names
                var customer_response = await this.customerService.GetAllMobilityCustomers();

            return ProcessServiceResponse(
                apiContext,
                service_response,
                items,
                null);

        }

This can be easily solved by allowing MARS in your connection string. Add MultipleActiveResultSets=true to the provider part of your connection string (where Data Source, Initial Catalog, etc. are specified). Sample connection string

 "CoreCS": "Data Source=xxx.xxx.xxx.xxx;Initial Catalog=Mobility;Integrated Security=False;User Id=sa; Password=xxxxxx;Max Pool Size=20; Connection Timeout=10;MultipleActiveResultSets=true;",