A terminal server can provide unlicensed client sessions for 90 days from the first connection. Once the grace period has expired, the terminal server must be able to verify the existence of a license by connecting to an activated license server. Every 15 minutes, the terminal server discovers and caches license servers.
If the grace period has expired, and a license server has recently come online, it may take up to 15 minutes for it to be discovered.
Until the license server is cached, clients are denied a connection.