VPN client connection works on one machine but not the other

I have a VPN connection to a remote server (on a Synology NAS) which is working fine on one of my Windows 11 PC's, but when I set it up in exactly the same way on a 2nd machine (also Windows 11) I get this error:

"the network connection between your computer and the vpn server could not be established because the remote server is not responding. This could be because of the network devices between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem"

The connection is L2TP/IPSec with PSK. I have entered exactly the same server name, PSK, Username and Password on both machines. Both are connected to the same LAN (one via ethernet, the other via Wifi). It's the machine on Wifi that can't connect. It's an Asus Zenbook S13 OLED.

The problem machine has full internet access, and can connect to a PrivateInternetAccess VPN without any problems. I always disconnect the first VPN connection before attempting to connect from this machine.

Any idea why the 2nd machine can't connect but the first one can?

itm60 said:

The connection is L2TP/IPSec with PSK

Click to expand...

IPSec means that IPSec rules need to exist?
Does the first machine (where it works) have them set up?

2nd question, does your VPN provider allow multiple connections?
3rd question, does connecting via wi-fi works if the first computer (where it works) is shut down?

I didn't explicitly set up IPSec rules on the first machine (I wouldn't know how to!)
I don't know if the VPN provider allows multiple connections, but I have only ever tried connecting one machine at a time (never both at the same time). The other machine can't connect even if the first machine is shut down.

Assuming everything is correctly set up the error message indicates you're not receiving response from VPN server which might mean that inbound traffic is blocked in firewall, not explicitly with firewall rule but implicitly with default block.

Try turning off windows firewall and try again.

2nd suggestion, can you test connecting the troubling computer via LAN cable to your router and see if that works?

I just tried connecting via ethernet instead of Wifi, with the Wndows Firewall turned off, and got the same error
???

There are only 2 possible causes I can think of:
1. The VPN server is indeed not responding
2. Or your laptop is not handling or dropping VPN responses.

To rule out 1st possibility you'd need to use process monitor and watch network activity to see if VPN is responding or not.
If it responds troubleshooting makes sense, otherwise you need to contact VPN service.

The server is the built-in VPN server on a Synology NAS. It's owned by a friend in the USA. He has sent me a screenshot of the log from the VPN Server, and it only shows successful connections (from the original PC). There is no trace of the failed connection attempts from my other machines.

itm60 said:

it only shows successful connections (from the original PC)

Click to expand...

I'd be curious how you determine from the log it's from original machine, based on:
1. private IP?
2. machine name?
3. public IP?

Can you please tell how is VPN set up on your laptop?
Is it set in "Network & Internet" in settings app?
If so make sure your internet connection on laptop is either not metered connection or VPN is allowed over metered connection.

I set it up from the "Network & Internet...VPN" screen. I selected these options:
- VPN Provider: Windows (built-in) - this was the only option available
- Connection name: Synology VPN
- Server name: the address of the VPN server (****.synology.me)
- VPN type: L2TP/IPsec with pre-shared key
- Pre-shared key: the PSK provided to me
- Username: the username provided to me
- Password: the password provided to me
The internet connection on the laptop is not a metered, or restricted in any way. I have this same problem with a 3rd (Windows 10) PC on the same LAN.

In settings app under Network & Internet -> VPN -> Your VPN when you click "Connect", upon failure to connect you should see yellow text explaining the error,
what does it say?
you can also take a screenshot.

The error message was as per the OP:
"the network connection between your computer and the vpn server could not be established because the remote server is not responding. This could be because of the network devices between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem"

Link below explains one fix, make sure you follow it exactly,
when you get to step where you need to type a value in registry set the value to 2


Let us know if this worked.

Thanks. I just tried adding that registry key but unfortunately it didn't help - I get the same error when I try to connect.

The only thing suspicious to me is that logs on server show only one computer connecting,
I would ask your friend to give me IP address, then you can set it manually in VPN settings in settings app under:
"advanced options" -> more VPN properties -> networking tab -> double click IPv4 item.

Alternatively on computer which works you can run ipconfig command in command prompt to see current IP and DNS of the VPN connection,
then copy IP and DNS address to configure VPN in before mentioned step.