If you have Windows Remote Desktop Services installed on your instance, then the conditions of the Client Access Licenses (CALs) are enforced. With these CALs, RDP connections will fail under any of the following conditions:
If your RDP connections fail, you can use the admin switch to connect to the instance for administrative purposes. This can be done on a Windows machine by using the native Remote Desktop Connection client.
To resolve issues with RDP connections, purchase new licenses for your instance. For more details about CALs, review the Microsoft documentation.
Alternatively, if Remote Desktop Services is not required, uninstall the service and use the regular RDP connections.