Hi Michael,
You should add post_logout_redirect_uri parameter to your end_session request. You can specify client post_logout_redirect_uri during client registration.
This is working tests which allow to see full workflow:
ttps://github.com/GluuFederation/oxAuth/blob/master/Client/src/test/java/org/xdi/oxauth/ws/rs/EndSessionRestWebServiceHttpTest.java
Regards,
Yura