How to Fix Windows 10 L2TP/IPSec Pre Shared Key Not Working (2025 Guide)

If your L2TP/IPsec VPN with a pre-shared key is not connecting on Windows 10 or 11, that can halt work fast. You might see the message “The L2TP connection attempt failed because the security layer encountered a processing error.”

This guide shows what is going on and how to fix it. Work through the fixes in order. After that, check the common reasons so you can prevent it from happening again.  You can also check the Windows 11 L2TP/IPsec troubleshooting guide for comparison if you use multiple systems.

Step-by-Step Fixes for L2TP/IPsec VPN Not Working on Windows 10

Try these one by one. Test the connection after each step.

1. Check Your VPN Settings

Go to Settings → Network & Internet → VPN → Edit.
Make sure VPN type is set to “L2TP/IPsec with pre-shared key.”
Re-enter your PSK, username, and password.
Verify the server address is correct and active.

For extra guidance, see how to connect to a VPN.

2. Start Required Windows Services

Press Win + R, type services.msc, and press Enter. Start and set to Automatic:

• IPsec Policy Agent
• IKE and AuthIP IPsec Keying Modules

Restart the PC and test again.

3. Allow VPN Ports and Protocols

On your firewall or router, allow:

• UDP 500 for IKE negotiation
• UDP 4500 for NAT traversal
• UDP 1701 for L2TP tunneling
• IP Protocol 50 for ESP

In Windows Defender Firewall, add inbound rules for UDP 500 and 4500 and a custom rule for ESP.

4. Enable NAT Traversal for VPN Connections

Open regedit and go to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
Right-click → New → DWORD (32-bit) Value → name it AssumeUDPEncapsulationContextOnSendRule.
Set the value to 2 and restart the computer. This allows VPNs to work through most routers and fixes many Error 809 cases.

5. Reinstall WAN Miniport Adapters

Open Device Manager → Network adapters. Uninstall:

• WAN Miniport (L2TP)
• WAN Miniport (IP)
• WAN Miniport (IKEv2)

Click Action → Scan for hardware changes. Windows will reinstall clean versions. If your VPN entry is missing from the list, use this fix for VPN not showing up in Network Connections.

6. Turn Off IPv6 Temporarily

Press Win + R, type ncpa.cpl, right-click your connection, select Properties. Uncheck Internet Protocol Version 6 (TCP/IPv6), click OK, and test again.

7. Pause Antivirus or Firewall Protection

Disable any third-party antivirus or firewall for a short test. If the VPN connects, add exceptions for UDP 500, UDP 4500, and ESP, then turn protection back on.

8. Check VPN Server Configuration

Confirm with your provider or admin:

• The server supports L2TP/IPsec with a PSK
• The PSK is correct
• Your account has permission

Try the same credentials on another device to see if the issue is local to your PC.

9. Remove Problematic Windows Updates

Open Settings → Windows Update → View update history → Uninstall updates. Remove updates installed right before the VPN stopped working. Reboot. Install newer fixes after the VPN works again.

10. Fix VPN IP Conflicts

Make sure your router’s subnet, for example 192.168.1.x, is different from the VPN’s internal network. Overlapping networks break routing. To confirm that internal resources are reachable, see how to access files through a VPN on Windows 10.

11. Reset Network Settings in Windows 10

Go to Settings → Network & Internet → Status → Network reset → Reset now. Reboot and set up the VPN again.

12. Update Network Drivers

Open Device Manager → Network adapters, right-click your Ethernet or Wi-Fi card, and choose Update driver → Search automatically.

13. Flush DNS and Reset Winsock

Open Command Prompt as Administrator and run:

ipconfig /flushdns
netsh winsock reset
netsh int ip reset

Restart the PC and test again.

14. Check the Event Viewer for VPN Errors

Press Win + X → Event Viewer → Windows Logs → Security or System. Look for Event IDs 20227 or 20209 for details on failed auth or missing services.

15. Create a New VPN Profile

Delete the old VPN profile and recreate it by hand. This clears hidden corruption or wrong parameters.

16. Change DNS Settings

If the VPN connects but browsing fails, switch to public DNS. Try Google (8.8.8.8, 8.8.4.4) or Cloudflare (1.1.1.1).

17. Update Router Firmware

Update the router firmware or enable VPN passthrough. Older routers can drop IPsec packets.

18. Test with a Different Network

Connect from another Wi-Fi or a mobile hotspot. If it works there, your main network or ISP is likely blocking IPsec.

Common Reasons Why L2TP/IPsec VPN Stops Working

• Wrong pre-shared key or credentials: One wrong character blocks the handshake.
• Authentication mismatch: The server may require a different protocol. See VPN protocols and authentication types.
• Firewall or NAT blocking: Missing UDP 500, 4500, or ESP permissions.
• Disabled IPsec services: IPsec Policy Agent or IKE modules are stopped.
• Corrupted network adapters: WAN Miniports damaged or missing.
• NAT traversal disabled: Registry key not set to AssumeUDPEncapsulationContextOnSendRule=2.
• Security software interference: Third-party tools blocking IPsec.
• IPv6 conflicts: Dual-stack routing issues.
• Windows update bugs: Some updates have broken L2TP/IPsec until later fixes.

Quick Reference: L2TP/IPsec VPN Error Codes and Fixes

Problem	Fix
Wrong PSK or login	Re-enter credentials and PSK
Error 809	Enable NAT traversal registry key
Error 789	Allow UDP 500/4500 and ESP, start IPsec services
Error 766	Switch to PSK instead of certificate
Connects but no traffic	Fix subnet conflict or turn off IPv6
Error 720	Reinstall WAN Miniports
Error 20227	Check IPsec Policy Agent and IKE modules

FAQs About L2TP/IPsec VPN on Windows 10

Final Verdict: Fixing L2TP/IPsec VPN Problems on Windows 10

Most issues come from a wrong pre-shared key, blocked ports, or NAT problems. Work through the fixes in order and test after each one. If the VPN still will not connect, contact your provider. The problem may be on the server side.