In our small network there is a Windows 2008 R2 Domain Controller that also serves as Remote Access Server. For years, we could access this server and the resources in the network over a VPN connection without any problem.

Since some time however, I am able to connect to the VPN, but my Windows 8 client (and another one I used for testing purposes) is not able to connect the domain controller afterwards. I can access any other server in the network, but there seems to be a problem regarding the trust between the client(s) and the server. If I connect the client to the network directly over a LAN cable, everything works as expected. Also I can connect to another server over VPN and open a RDP prompt to the DC without a problem.

On the client, whenever I try to access the DC, I get an access denied message. I've tried to update the group policies both over VPN and LAN.

Also, I've removed the client from the domain and re-added it. The client shows a message that Windows requires valid login information when connected to the VPN - but my credentials are valid. They work when I logon to the client when not connected to the VPN and also when connected to the LAN.

Turning off the firewall on the client and the server did not change anything.

DNS resolution works both on the server and the client.

What else can I do to diagnose and solve the problem?