Using V1 API we get 400 Bad Request returned from server when attempting to upload a new contact individual using CreateNewContact API. I have a suspicion this relates to the problem I reported recently in item titled..."using version 1 api Not able to retrive more than 50 contacts from Do Not Mail list.".
400 error when creating a Contact is usually related to a data issue. Have you looked at the returned error message to see what we are saying is wrong? 400 Bad Request is a data validation failure and we usually return a detailed error of what the problem was, or at least which node had the bad data. If you could share your code, that would help as well.
Dave Berard Senior Product Manager, Constant Contact