Please disregard my previous answer, I misread your question and decided you are trying to request your newly registered client's configuration properties via OpeinID Connect Registration extension (which actually works as it should). Now I clearly see you are trying to access Gluu's SCIM api. Please accept my apologies for providing you misleading information.
At the moment, after I've tried to follow SCIM api guide in the Gluu wiki, in an attempt to make sure it is indeed functional before trying to reproduce your issue, and failed, I'm trying to get more info about its current state from the dev team. As soon as I have some news I'll let you know for sure.