I've got a VPN L2TP/IPsec server behind a NAT-T device. I had issues with connecting windows clients to the server but that was resolved via adding a registry entry "AssumeUDPEncapsulationContextOnSendRule". As per this link.
The next issue is that although connected, the windows clients cannot access any web interface of devices on the VPN network. For example, 192.168.2.1:80 (routers interface), as well as access to the NAS interface and controllers. However the windows client can access SHH connection to devices, as well as any networked shared drives from the NAS, it's only web interfaces/traffic that is not working.
If I connect the same windows client via a PPTP link to the same VPN server, there is no issue with access. I've confirmed it's not an issue on the server/router side by using an iOS device to connect via L2TP. The iOS device can access files, web interface and internet when it is connected.
Are there any settings on the client that could prevent browser traffic from using the VPN? The windows clients cannot access the internet via this VPN connection either.