Go to the DC, open taskmanager...select users tab. You should be seeing a 'disconnected' user. Select and logoff that user. You only have 2 'free' terminal services connections on the DC. As long as the 'domain admin' and the remote desktop user log off any sessions instead of just close the window...both users should have access. Closing the window disconnects and maintains one of those two connections. If you want more than 2 connections, you will have to purchase and install the licenses for terminal services.
