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I have a problem installing a L2TP VPN Connection with Windows Server 2019.

There are two interfaces: "Intern" - 10.0.0.1 / Internal interface "Extern" - 192.168.2.3 / External interface, connected through router to the internet

I can easily connect to the VPN from a local PC, in the same network, e.g. 10.0.0.10. Connection can be established with the correct user and PSK.

But when I try to connect to 192.168.2.3 (from the 10.0.0.10, 192.168.2.3 is accessable from this IP, because of routing) or from external (internet, correct port-forwarding in Internet-Router (FritzBox)), I just get no connection established.

The only thing is the NAT between it. The client is correct configured, included the registry values HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPSec and HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent

What am I doing wrong? Firewall cannot be the problem, because I deactivated the firewall for testing purposes.

Maybe somebody has an idea that could help. Thanks very much in advance.

EDIT: The ISAKMP packets are received, but there is no response from the server (see wireshark screenshots). The firewall dropped nothing, because it was disabled for test purposes.

Screenshots of the configuration: External Interface Routing Settings Internal Interface Routing Settings External Interface Port-Forwarding Settings Routing settings Routing security settings Interface list enter image description here

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The problem was, that there was port-forwarding in NAT-interface. The forwardings must be disabled (although they was to 127.0.0.1).

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