We have a test contact that has no data (name/address etc.) associated with it. It is also in the opted out state. I've been having a lot of problems updating this contact - even just retrieving the data and posting it back does not work.
I either get a bad request error or a "there was a conflict between the supplied data and the existing resource" error. These responses are really unhelpful and make debugging hard work!
The simplest method of getting the contact and then posting it straight back has PUT JSON that looks like this:
I'm sorry that you are now experiencing this issue with the API. We are currently looking into the issue, trying to reproduce it on our end. We'll let you know how things are going, and definitely get back when figure out where the issue may be.
Rich Marcucella Sr. Technical Writer, Content Developer Web Services Team http://developer.constantcontact.com