Unable to connect to L2TP/IPsec vpn from Windows 10

Question

When I try to connect to my L2TP/IPsec vpn with pre-shared key, I get the following error:

The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer

Things I have done to try to fix it:

I checked that the correct ports are forwarded on my router.
I checked the pre-shared key and account password.
I restarted the VPN on the server side and made sure the right pre-shared key was set.
I checked if the IKE and IPsec services were running
I tried to restart the IKE and IPsec services
I checked in the network adapter that under security the MS-CHAP v2 is allowed
I checked in the network adapter that under options, the PPP settings have LCP extentions enabled

These are the only things I could find to fix this issue, but they didn't help me resolve the problem. Any tips on how to fix this?

Turn logging on and check basic things: connection to host VPN, phase 1 connection, phase 2 connection. — anon, Commented Jan 31, 2021 at 21:52
if your PC is behind the NAT (Router), you might need to create AssumeUDPEncapsulationContextOnSendRule key in registry. For more details , you can refer to the following article: docs.microsoft.com/en-us/troubleshoot/windows-server/networking/… — Candy, Commented Feb 1, 2021 at 8:28

coolcat007 · Accepted Answer · 2021-02-11 10:35:23Z

I found out that the issue was that I tried to connect to a VPN that runs on the same local network, using the public ip address.

I found this out when doing some experiments with my phone:

When I try to connect to the VPN using my phone on 4G using the public ip address, I have no issues connecting.
When I try to connect to the VPN using Wifi on the same network using the public ip address it does fail to connect.
When I try to connect to the VPN using Wifi, but using the local ip address, then there also is no issues when connecting.

On my PC using the local address also fixed the issue.

I don't know why this causes issues, but at least I now know how to fix this.

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