The list of the licensed hosts can be reset by restarting syslog-ng.
1. Restart syslog-ng.
Check the knowledge article "Managing and checking syslog-ng service on Linux" for more information.
2. Get the list of licensed-hosts. It may happen that some clients reconnect immediately and a non-empty list is returned.
See "How to create a list of all the hosts reporting into a PE server" for more information.