We don't exactly know what happened, but the oxauth log said ClientAssertion.java:49 -> cannot verify JWT, and ClientAssertion.java:130 -> JWT has expired
From one day to another, it stopped creating or updating users. Of course, we kind of mixed dev and test environment users, and after we cleaned up (deleted all users), the error disappeared.
Maybe the session token for that registered client in Gluu expired but it was still requested for authentication requests.