Do you have the errai-jaxrs-provider.jar deployed to your server (it's a jaxrs provider that ensures your server is producing Errai's JSON format)?
If you prefer to use a Jackson generated JSON format then you don't need to deploy the errai-jaxrs-provider but you need to configure the client to expect the Jackson format:
We just figured out this to be the issue. We previous had:
b. we included errai-jaxrs-provider-2.4.1.Final.jar
The only thing that worked was when we removed a. and included b. in the WAR.
OK, great! In order for a) to work (if you ever wanted to) you will need to make sure your server is returning JSON generated by Jackson. When using RestEasy you need to deploy the resteasy-jackson-provider. By default it will use Jettison to generate JSON.