Hello,
Error code 789 typically indicates a problem with the L2TP/IPSec connection. Here are a few troubleshooting steps you can try to resolve the issue:
1. Verify the VPN server settings: Double-check the L2TP/IPSec VPN server settings on your Synology NAS. Make sure the server address, authentication methods, and shared secret match the settings on your Windows 11 client.
2. Check firewall settings: Ensure that the necessary ports and protocols are allowed through both the Windows Firewall and any other firewall software or hardware that might be blocking the VPN connection. The required ports for L2TP/IPSec are UDP 500, UDP 4500, and ESP (IP protocol 50).
3. Disable antivirus and security software temporarily: Some antivirus or...