Thank you for your message.
On which operating system is the Host installed?
You have the enhanced multi user support on a server- that means that you are able to connect to the server-ID and to the user-ID.
The user-ID is the ID which you get, when you are connected to the server via a RDP session - this ID is available as long as the RDP connection is open.
When this connection is closed, you are not able to reach this ID.
The server-ID is the ID which is always available. When TeamViewer is open on the server, please click on the blue sign right next to the ID. You will get an info text in which you can see which ID the server-ID is.
Please make sure that you connect to the server-ID.