I recently ran into this message when trying to activate a UAG DirectAccess box after changing the public IP addresses. It immediately rang a bell, as I have written before about the “Forwarding on the 6to4 network interface cannot be enabled” and how that one is usually corrected by heading to Device Manager and deleting out the multiple instances of the 6to4 adapter that have plugged themselves in there. However, when receiving “Forwarding on the Teredo network interface cannot be enabled” you would follow the same logic, head over to Device Manager, and discover that there is only one Teredo adapter listed. Furthermore, deleting that adapter and restarting the server does not correct the UAG activation error message.
The inability to enable forwarding on the Teredo interface seems to be related not to having multiple adapters, but rather some confusion between the Teredo adapter and the SSL Network Tunneling adapter. If you take a look at “ipconfig /all” on your server, you will likely notice that there are two interfaces with the same name – “Local Area Connection* 9” – one of these is your Teredo Tunneling adapter, and the other is your SSL Network Tunneling adapter.
The solution: Uninstall the SSL Network Tunneling adapter from Device Manager, and restart the server. UAG should now activate successfully.
Before resorting to doing this I tried running many commands and different things to force the enabling of forwarding on the interface, but nothing I tried worked. Ultimately, deleting the SSL Network Tunneling interface will not negatively affect a DirectAccess system, because the SSL Network Tunneling adapter would be used by the Network Connector VPN, and it is not supported to run Network Connector VPN and DirectAccess connections on the same UAG server. Maybe this adapter confusion trouble is part of the reason why. After deleting the adapter and restarting, UAG was able to activate successfully and both DirectAccess and SSTP VPN connections (which are supported to run at the same time) are working great on this machine.