Although you're most likely debugging from your local machine you'll want to make sure you update your app on the server. For starters the "hosted.html" that is generated in 2.0 has specific information that facilitates communication between the Developer Plugin and your Code Server.
Typically you don't need to worry about these files since they are auto-generated and picked up by the embedded Jetty browser that ships with GWT. But given that you are deploying and debugging via an external server, you will need to take the added steps of recompiling with 2.0, deploying once, and then debugging.