When installing the Remote Desktop Session Host role on a Windows Server, you get a 120 days grace period where you do not need to specify a license server and/or have licenses installed. After the grace period, it is no longer possible to log in with even a single session through RDP, unless you use the admin console (/admin option). Luckily, you can reset this counter by getting rid of a specific key which gets created at the start of the grace period, effectively resetting the grace period counter. This can even be done if the grace period isn't completely over.

Log in directly on the server (or use the /admin option) and fire up regedit.

Navigate to the key: HKEY_LOCAL_MACHINE:\SYSTEM\  ...