I've seen similar behaviour as pak
describes, but in W2k8 R2. After logon, the client usually loses connection with server, and is unable to reconnect. Sometimes it does not lose connection completely, and I can move the mouse on server side, but no client screen updates are made, so I cannot see what is going on remotely. If I disconnect the session, then I'm unable to reconnect.
I ended making a script to kill a frozen server, and restart it when any user log in, so I get a clean server connection after loging in. You can see it here: https://forum.ultravnc.net/viewtopic.php?f=71&t=31909#p100893
If you configure the client to autoreconnect, then it will reconnect after logon, cleanly.