The error message most likely is coming from oxAuth, so you need to look under `/opt/gluu/jetty/oxauth/logs/`, specifically check file named `oxauth.log`. Try to flush all cookies related to this host and log in again. If you try to log in to admin web UI, and see this error, then it's extremely strange, as under no normal circumstances such thing should happen. Can it be you accidentally removed the system client entry oxTrust uses, or changed its properties?
I would suggest to consider reinstallation if it's a fresh instance, as unless you have any recent backup for this host, you may be in a lot of trouble trying to fix it.