Once an instance of HelpNDoc is activated, it will connect to the license server from time to time (usually, every 90 days) to check that the license is still valid. If it can't connect to the license server, it will keep working for a certain period of time, the grace period (usually, 14 days).
To fix that, make sure that the computer running HelpNDoc is connected to the Internet and that the following web sites are accessible and white-listed on your router / proxy / firewall: