Hello. Can you please provide the company id, the invoice number for a failing request, and a timestamp of the request (UTC or with the timezone)? We believe that the cause of this is that the RELEASE call is asynchronous. You get a 200 response immediately but the release process continues. When the POST of the attachment comes to the API, the release is not yet finished so the two processes run in parallel, and the exception is triggered. You could try to get the status of the invoice and see if that status is "Open". Only when the status is "Open" you will send the attachment. We are also investigating the proper solution for you to further implement this IF the above assumption proves correct. ... View more