Thanks for contacting support.
Please make sure that you are setting license correctly before using any feature/method of the API. In case if you still face any issue, please share some more details like your application type and your way to set the license. This would help us in investigating the scenario in our environment and address it accordingly.
Furthermore, each license file has a validity of one year of free upgrade and the license expiry date is specified in
<SubscriptionExpiry>20170825</SubscriptionExpiry> tag, where 2017 is year, 08 is month and 25 is the date of month. You can upgrade to any/latest release versions of API published before license subscription expiry.
However please note that the license file can be used for as long as you want (unless you are using it with API versions published before license expiry).