The terminal server has exceeded the maximum number of allowed connections.
You can help prevent this from happening by setting a policy on the server to automatically disconnect when idle.
To change this setting, go to Administrative Tools \ Terminal Services Configuration.
Click on Connections in the left hand pane, and then right click RDP-Tcp and select Properties. In the resulting window select the Sessions tab.
Check the boxes for “Override user settings” and change the idle session limit to something reasonable, like an hour. You can set it lower if you’d like.
Change the radio button to “Disconnect from session” when session limit is reached. This will make all sessions automatically mark as disconnected on the server. The session will be saved exactly as it was, but the server will mark it as disconnected so that you can log back into the session again.
