I am also facing the same problem "Error - Invoices already submitted". I went to the sales tax office in Kolhapur, Maharashtra. They told me that many taxpayers are getting the same issue and they are taking up the matter with GST team. They have asked me to mail them the screenshot of error, so that in case the issue is not solved by 5th, we are penalised. As at this time - 4th Sep 2017, 17:39, the problem still persists. If any one is able to resolve the issue, kindly post in this forum.