windows 10 L2TP/Ipsec VPN with synology NAS not working

Question

I am using a Synology NAS that has an L2TP/IPsec VPN server running. The NAS is located in a network behind a FritzBox router, and I am in another network that is also established behind a FritzBox router. From here, I can connect to the VPN via Debian and Android, but not from a Surface Pro 4 (up-to-date).

How do I fix the inaccessibility on the Surface Pro 4?

• Added: AssumeUDPEncapsulationContextOnSendRule with DWORD value 2:
  HKLM\SYSTEM\CurrentControlSet\Services\PolicyAgent
• Services Running:
  IPsec Policy Agent & IKE and AuthIP IPsec Keying Module
• Network Adapter:

I verified the credentials several times (username, domain, password, preconfigured key, etc.), but I still receive this error, with no error code I can follow:

The network connection between your computer and the VPN server could not be established
because the remote server is not responding.

This could be because one of the network devices (e.g. firewalls, NAT, routers, etc.)
between your computer and the remote server is not configured to allow VPN connections.

Please contact your Administrator or your service provider to determine which device may 
be causing the problem

---

I have tried the following, with none resolving the issue:

• Device Manager > Network adapters > Delete all WAN Miniports > Action > Scan for hardware changes
• Disabled Xbox Live-Networkservice (not running)
• Restarted DHCP-Client, but permission denied with code 4, so I could not restart.
  Trying to fix this permission issue did not work (more permissions in the registry and adding some extra users to the Admin group like System and so on), so I tried other things.

What am I missing? Why is this setup working on several Windows 10 computers, but not on this one Surface?

Solution

Ok, I solved it accidentally.
Windows has problems when there is a manual DNS configured on the network adapter.

So the following fixed it for me:
Win + R -> ncpa.cpl -> right click on the network adapter that is used for the internet connection (wifi or lan) -> settings -> (in my case additionally disabling IPv6) -> click on IPv4 -> settings -> then DNS-Serveraddress automatically and not manually

If there are still problems, temporary disable the adapter Npcap Loopback Adapter if present.

Comments

[christopher2007]

hi, thanks for the answer. But unfortunately I am using 1803. And uninstalling also had no effect. But worth a try :)

[christopher2007]

also no effect.