L2TP authenticated, then terminated

RouterOS General
1

RouterOS 6.47.10 on HAP AC2. I have an L2TP server on that. There are multiple Windows 10 clients connected to it. One of the clients suddenly stopped working. The others are okay.

This is what I see in the logs when I try to connect from that client:

07:55:15 ipsec,info respond new phase 1 (Identity Protection): router.ip.no.telling[500]<=>client.ip.no.telling[13700]
07:55:15 ipsec,info ISAKMP-SA established router.ip.no.telling[4500]-client.ip.no.telling[4500] spi:36dba7f018a179cb:9b80b83af50a6ab9
07:55:15 l2tp,info first L2TP UDP packet received from client.ip.no.telling
07:55:16 l2tp,ppp,info,account stonemining logged in, 10.14.200.107 from client.ip.no.telling
07:55:16 l2tp,ppp,info <l2tp-stonemining>: authenticated
07:55:16 l2tp,ppp,info <l2tp-stonemining>: terminating...
07:55:16 l2tp,ppp,info,account stonemining logged out, 0 0 14 0 2 from client.ip.no.telling
07:55:16 l2tp,ppp,info <l2tp-stonemining>: disconnected
07:55:16 ipsec,info purging ISAKMP-SA router.ip.no.telling[4500]<=>client.ip.no.telling[4500] spi=36dba7f018a179cb:9b80b83af50a6ab9.
07:55:16 ipsec,info ISAKMP-SA deleted router.ip.no.telling[4500]-client.ip.no.telling[4500] spi:36dba7f018a179cb:9b80b83af50a6ab9 rekey:1

I have removed the L2TP connection on the client, and re-created it. Double checked the shared key and also the user secret. Actually the passwords/secrets cannot be bad because the client is authenticated. But then it disconnects - but only on this client.

The problem must be on the client side, because the very same server with the very same settings is working for many other clients. (The only difference is that they use different /ppp secret entries. But they are the same Windows 10 OS, all updates installed and rebooted etc.)

On the client side, all I see is this error:

A connection to the remote computer could not be established. You might need to change the network settings for this connection.

I have no idea what to do or how to fix it. This error message is garbage, it does not tell what is missing exactly, or what went wrong.

2

I remember someone here to recommend to completely remove the VPN configuration at Windows side and start from scratch, or even to remove all VPNs - try to find that post with the details on what exactly needs to be done as it did help the OP. Can’t give you more clue unfortunately.

3

Maybe a Windows problem…

try this

https://docs.microsoft.com/en-GB/troubleshoot/windows-server/networking/configure-l2tp-ipsec-server-behind-nat-t-device

4

Go to Windows Settings - Event Viewer – Windows – Application
Look for the log entry for the Remote Connection. If error code is 720, then
Device Manager – Network Adapter
Remove all WAN Mini port Drivers ( Uninstall ).
Reboot and try reconnecting

5

Yes, error code was 720. Removed all miniport devices, rebooted the system and now it works.

Thank you!

Side note: I hate when Windows works like this: there is a misterious error that cannot be explained, and cannot be prevented. Just “fix when it happens”. :-@