Win7 cannot establish IPv6 L2TP tunnel to SoftEther Server

[wangqiaoqian]

Hello，I use windows7 (by adding a native L2TP VPN) to try establishing IPv6 L2TP connection to the SoftEther server on Centos7, but always failed with "Error 1168：Element not found ....".

However:

(1) the same windows7 can establish L2TP connection by adding a native L2TP VPN with the same L2TP server using IPv4 address without any problem.
(2) the same windows7 can use SoftEther client to connect to SoftEther server using IPv6/IPv4 address without any problem.
(3) windows11 and windows10 can connect to the same IPv6 L2TP server by adding a native L2TP VPN without any problem.

Has anyone encountered this issue, and maybe I missed some configuration or are there some workaround to fix that? Thank you very much in advance.

The configuration for Windows is as follows:
（1） VPN type: L2TP/IPSec
（2） Requires encryption (will disconnect if server declines)
（3）CHAP and MS-CHAP v2 are checked
（4）The correct pre-shared key is entered
（5）The correct username and password are set.

There is no firewall between the IPv6 L2TP/IPSec server and the Windows PC; the server is listening on the ports：
TCP 443
UDP 500
UDP 1701

I have made the following attempts on Windows 7：
（1）Execute ‘sfc /scannow’ with administrator privileges, output no error:
“Verification 100% complete.”
（2）The online solution using the DISM command doesn't apply ，because Windows 7 does not support DISM.
（3）Add key ‘AssumeUDPEncapsulationContextOnSendRule’，value = 2
（4）I referred to the method in this link, but there is no ProhibitIpSec in my registry：
https://www.vpnusers.com/viewtopic.php? ... ows7#p6478