How to resolve L2TP VPN connection error on Windows Server 2025

Question

A remote VPN connection has been set up on Windows Server 2025, but connection attempts from a different PC on another network result in the error: 'The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.' All required ports (115, 4500, 500, 1701) have been opened and forwarded. What possible solutions are available for the client or server?

Answer 1

Hello,

 

Thank you for posting in Q&A forum.

To further troubleshoot this issue, please kindly try below steps:

1.Check Firewall Settings and ensure that the related ports (500, 1701, 4500, 115) are open on both the server and the client. Additionally, ensure that the firewall is not blocking VPN traffic.

2.Ensure that the correct protocols (L2TP, IPsec) are enabled and the pre-shared key is correctly configured.

3.Ensure that both the Windows Server 2025 and the client PC have the latest updates installed.

 

I hope the information above is helpful.

If you have any questions or concerns, please feel free to let us know.

 

Best regards，

Jill Zhou

 

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