What it looks like is that the Accept header is asking for form data, but the server can only give you JSON. What needs to happen is that the call through RestTemplate needs to have an Accept header that includes "application/json". I'm a little surprised that the header wouldn't already include that, but that appears to be the problem.
I'm sorry to say that I don't have a handy example of using Spring Social with S2OAuth. From what I understand, the specific use case you're looking at is to have a Spring Social consumer connect with a REST API that is secured with S2OAuth as an OAuth provider? Is that right?
Greenhouse exposes a REST API that is secured with S2OAuth and at one time (a long time ago) there was an experimental Greenhouse module for Spring Social. Although it worked, it was just an experiment and we ditched it. In the time that has passed since then, S2OAuth has changed a lot (Greenhouse hasn't even caught up with the latest changes) and so has Spring Social...so I wouldn't be surprised to find out that one side or the other (or both) have shifted to not play well together.
I can't offer any quick answers at the moment, but there are plans in the near future to bring Greenhouse up to speed with the latest S2OAuth work. When that happens, I'll be sure to set aside some extra time to build out a Spring Social sample to connect with Greenhouse.
Spring Social Project Lead