Encounter errors when execute batch request.

Hi Experts,
We use OAuth2 connection in our code and one of our customer create 5 invoices in a batch request. Recently they always encounter errors like:
1. Intuit.Ipp.Exception.IdsException: There was an error while communicating with the Ids Server. ---> Intuit.Ipp.Exception.CommunicationException: The Response Stream was null or empty.
   --- End of inner exception stack trace ---
   at Intuit.Ipp.Core.CoreHelper.CheckNullResponseAndThrowException(String response)
   at Intuit.Ipp.DataService.Batch.Execute()
   at Tc.Int.Platform.BSPQuickbooksOnline.BSPQuickbooksOnline.ExportTransactions(XmlNode xmlRequest)
2. Intuit.Ipp.Exception.ValidationException: Validation Exception was thrown.Details:Business Validation Error: The action could not be completed because another user was creating, editing or deleting a transaction with inventory products at exactly the same time. Please try again in 30 min. If the issue persists, please contact Customer Support..
Exception Detail: Business Validation Error: The action could not be completed because another user was creating, editing or deleting a transaction with inventory products at exactly the same time. Please try again in 30 min. If the issue persists, please contact Customer Support.

We tried to disconnect and connect again. Sometimes the request works for 1, 5 or 10 invoices but later the error persists. Could you help to check what's the root cause?

Thanks,
Steven