I hope that you have resolved this, But I had a similar 400 error,
but I have resolve this by making sure that I always start my Jboss server from the same directory. I you install the default installation package and use "ant start jboss" when you first test the installation and load data into guvnor, then there are a good chance that you will see this error when you later start the jboos server through another means, from with-in eclipse or directly in say ${jboss.home}.bin
The reason is that Guvnor install the repository.xml and repository in the start-up directory. This can easily be fixed in the build.xml file by adding the dir="${jboss.home}/bin" in the ant exec command.
I hope this help