VNC Connect Enterprise subscriptions include VNC Server version 5.x keys, to allow you to license VNC Server 5.x.
All VNC Server V5.x keys generated by the VNC Connect service (accessible from the Deployment page of the VNC Connect portal) will show an expiration date of 2016-10-30 as they are "legacy" keys.
So long as you have a valid VNC Connect subscription, the key is valid.
You will need to ensure you are running the latest version of VNC Server (5.3.3) which doesn't display a message about the expired license.
You can download VNC Server 5.3.3 from the following locations:
Licensing VNC Server V5.x
To apply your V5.x license key to VNC Server 5.3.3, see Updating an Enterprise License key
For information on licensing VNC Server on legacy platforms, see https://help.realvnc.com/hc/en-us/articles/360006435478