I find it hard to say whether the logs say something useful, because it could be unrelated, but I do have several of these error messages in the logs: (restart_authentication or login errors)
but with different fillings for the "error" field: already_logged_in (I didn't get that one before the upgrade, or not with this frequency)
, invalid_user_credentials (the latter, sure, but that won't get back to the client.)
2024-09-26 21:58:51,313 WARN [org.keycloak.events] (executor-thread-1168) type="RESTART_AUTHENTICATION_ERROR", realmId="xx", realmName="xx", clientId="xx", userId="null", ipAddress="xx", error="already_logged_in", response_type="code", redirect_uri="xx", redirected_to_client="true", response_mode="query"
2024-09-26 21:56:10,949 WARN [org.keycloak.events] (executor-thread-1079) type="LOGIN_ERROR", realmId="xx", realmName="xx", clientId="x", userId="null", ipAddress="xx", error="already_logged_in", response_type="code", redirect_uri="xx", redirected_to_client="true", response_mode="query"