To reproduce this issue on 22.214.171.124:
1. Set up a mapping from managed/user to an external target. Enable implicit sync and queued sync.
2. Configure selfservice registration.
3. As openidm-admin user, create a new managed user resource, it is successfully synced to external target.
4. Register a new user via self-service registration service, including clicking the validation link in email.
5. The exception is logged
6. The event is stuck in sync queue