W10 VPN Error: The L2TP connection attempt failed because the security layer encountered a processing error during the initial negotiations with the remote computer

Hello! My name is Cristiano. I'll try to help you.

The error you are seeing indicates that there is an issue with the negotiation process between your computer and the remote VPN server. This can occur due to a number of reasons, including misconfiguration of the VPN settings, network connectivity issues, or compatibility problems with Windows updates.

Since you have already ruled out the possibility of the KB5009543 security update causing the issue, you can try the following steps to troubleshoot the problem:

1. Check your VPN settings: Make sure that your VPN settings are correct, including the server address, username, and password. You can also try changing the VPN protocol from L2TP to another protocol like PPTP or OpenVPN to see if that resolves the issue.
2. Check your network connectivity: Ensure that your computer is connected to the internet and that there are no issues with your network connection. You can try resetting your network settings by running the following commands in Command Prompt:

ipconfig /release

ipconfig /renew

netsh winsock reset

netsh int ip reset

3. Update your network drivers: Outdated or corrupted network drivers can cause connectivity issues. You can try updating your network drivers by going to Device Manager and selecting the network adapter, then clicking on "Update driver".
4. Disable your firewall/antivirus: Sometimes, your firewall or antivirus software can interfere with your VPN connection. Try disabling them temporarily to see if that resolves the issue.
5. Contact your VPN provider: If none of the above steps work, contact your VPN provider for further assistance. They may be able to provide you with more specific troubleshooting steps or recommend a different VPN protocol to use.

I hope these steps help you resolve the issue. Let me know if you have any further questions.