The Default setup in Windows Server 2008 R2 and Windows Server 2012 Standard is to only allow a single session per user. This means that two different connections to the machine as the Administrator user are not allowed. This setting can be changed by logging into the 2008 server and accessing Start > Administrative Tools > Remote Desktop Services > Remote Desktop Session Host Configuration
Click Start > Log Off. If you just close the window, the session still remains and if you have reached the maximum number of concurrent connections, you will not be able to login. You will likely see a message like the one below.
The table below outlines the number of desktop connections that can be active concurrently.
|Operating System||Max Concurrent Connections|
|Windows Web Server 2008 R2||1 (cannot be increased)|
|Windows Server 2008 R2||2 (only single session per user)|
|Windows Server 2012 Standard||2 (only single session per user)|