OpenVPN Access Server 2.5.2 on Ubuntu 16.04 can no longer connect to the OpenVPN licence server (as of some time in 2023). This can be temporarily fixed by upgrading to 2.8.8 which still works (as of 2023-12-30 at least). According to OpenVPN's Keeping Access Server Updated, the openvpn-as package is not installed through the repositories until 2.7.5. If you happen to be on an earlier version, say 2.5.2, you need to setup the repository. The instructions for Ubuntu 16.04 are out-of-date if you can find them.
NOTE: All commands are assumed to be run as root. If not running as root, use the sudo command where appropriate.
Download the OpenVPN repo signing key (The curl command can be used, but only wget was available on my machine):