Occasionally, I get license expired even if it is valid. Why?

We have observed similar behavior, when “Treat offline as expired” license property is enabled. Sometimes, you won’t receive response from remote server. It can be due to remote server outage or because you have reached its limit of requests per specific time period. The best solution for that is not to use this feature or use your own timeserver. TLL accepts any standard NTP server you choose. Even if "Treat offline as expired" is not enabled, TLL tries to retrieve online time first and only if failed, it uses local computer time.