Terminalserver 2008 R2 random disconnects

Hi guys

I found alot stuff over disconnect errors on W2k8 R2 but nothing exact like my issue. First the environment:

Terminalserver: W2k8 R2 running on vSphere 5.1 U1. Single TS with about 25 users.

Clients: HP ThinClients, some with ThinPro, Smart Zero, Windows 7E, all clients have the same issues

Now to the issue: RDP connection to the server works always. But during work clients will face random disconnects from the server. The connection is reestablished within a second or two. But it is happening ranging from every 30 minutes to every 2 minutes which makes it really hard for users to work.

I have no errors in the event log. The only hints i have are the logon events telling me that a client disconnected and after a few second the event, that the client connected. (Event-ID 23, source: TerminalServices-LocalSessionManager)

Another thing is, that when I am using our VPN to the customer and connect via RDP to the same terminal server the issue never happens. It only happens to the ThinClients.

I really have no idea what else to look for and would appreciate any hints.

Thanks alot and best regards!