The allowed amount of hosts by the license is exceeded because of various reasons, like:
an issue in the log sending infrastructure filled up the list with unexpected host names,
already removed hosts kept in the list reserving the place in front of current legit log hosts,
The licensing subsystem keeps track all the hosts that are sending messages directly or being present as log sender hosts in the log messages. The list is ever increasing and has the new hosts appended regardless of the time of the last successful log retrieval.
The license counter can be reseted by restarting the underlying syslog-ng process on the appliance.
This can be achieved two ways by either
navigating to Basic Settings > System > Service control and pressing the "Restart syslog-ng" button.
rebooting the appliance (not recommended in this case)