L2TP VPN connections not working on MacOS Sonoma

Your post is indicating you have macOS 14.1. If that is true, you should upgrade to 14.1.1 and then test. If you already did do that, try booting into safemode then after that, reboot again and login normally to test it out.

Start up your Mac in safe mode - Apple Support (English)

OK. The remedy for me was once the VPN was connected, open Network Settings again and click on the button with 3 dots and a down arrow and choose Set Service Order, then drag the VPN connection above the Ethernet/wifi items.

To get VPN to work, I needed to promote the VPN connection above my Internet connection, which I could do up to Ventura. Sonoma doesn't show the full list when I click on the network button at the top of the screen, so I can't change the order to get VPN above Internet.

I'll need to downgrade to Ventura to get my VPN to work.

I have the same problem M3 PRO Sonoma 14.1.2

 l2tp_get_router_address 192.168.10.1 from dict 1

 l2tp_get_router_address 192.168.10.1 from dict 1

Thu Dec 14 22:13:06 2023 : L2TP connecting to server '***********' (***********)...

Thu Dec 14 22:13:06 2023 : IPSec connection started

Thu Dec 14 22:13:06 2023 : IPSec phase 1 client started

Thu Dec 14 22:13:06 2023 : IPSec phase 1 server replied

Thu Dec 14 22:13:36 2023 : IPSec connection failed

Thu Dec 14 22:14:43 2023 : publish_entry SCDSet() failed: Success!

Thu Dec 14 22:14:43 2023 : publish_entry SCDSet() failed: Success!

I'm having the same problem. Just bought a brand new MacBook Pro M3 Max running Sonoma 14.3. Setting up my BuiltIn VPN connection, L2TP over IPSec and it refuses to connect to my office VPN. I've got a old iMac with High Sierra (10.13.6) on the same home network using the same VPN config and credentials and that connects perfectly. It would be great if Apple can investigate this or have some suggestions on trouble shooting and fixing this issue. Or indeed if anyone else out there can help. It would be much appreciated.