L2TP - immediate disconnection upon establishing connection

[schoolcomputergeek]

We have a customer using Untangle and I have previously had L2TP / IPSEC VPN working fine for me to connect in to their site. Now getting the following error as soon as we connect:

Aug 15 10:59:23 alfr-gateway xl2tpd[112322]: Terminating pppd: sending TERM signal to pid 112599
Aug 15 10:59:23 alfr-gateway xl2tpd[112322]: control_finish: Connection closed to [ipaddress], port 1701 (), Local: 53900, Remote: 32
Aug 15 10:59:23 alfr-gateway xl2tpd[112322]: call_close: Call 3583 to [ipaddress] disconnected
Aug 15 10:59:23 alfr-gateway xl2tpd[112322]: child_handler : pppd exited for call 1 with code 11

I've replicated the settings in Windows 10 VPN client and Untangle exactly with my home instance of Untangle, and I have no problem connecting to VPN at home, so doesn't seem to be a Windows VPN issue.

Likewise I'm able to connect to my home instance VPN using my iPhone, but not to this customer site.

I have checked and double-checked the configuration of the customer site (L2TP/IPSEC as well as Local Directory) and reconfigured from scratch, deleting the Local Directory user and recreating, but still get the same problem.

When you connect using Windows 10 (1903) you get "The PPP link control protocol was terminated". It prompts for username and password again, which I enter correctly, then the VPN settings screen disappears. This happens every time unless I cancel out of the username/password prompt screen.

However, given I'm able to connect to my test instance at home with no issue, I think the problem is on the customer's untangle instance.

[jcoffin]

Look to see if the MTU from the ISP modem has changed. Also it could be anything along the way. I find OpenVPN far more robust in staying connected to an endpoint.