Error 131 on a single user

[AdminBMQ]

Hello,

Our SoftEther VPN Manager is installed on a Windows virtual machine with a single VPN Server and three virtual hubs. Each virtual hub manages its users and its DHCP with separate network addresses (VLANs). All users have a Softether Client software installed on their Windows systems, with settings adapted to the specific hub they should use. The VPN Server is accessed through a public address with a registered domain name (io.mydomain.com)

All is perfect except for a single user.

Recently he got a message when connecting :
Error (Error Code 131):
Several VPN Servers on the same IP address.

I can't manage to find why it happens. I checked if he would use a proxy, shut down the local firewall, looked for an additional virtual network, reinstalled the Client... Just to check as I recently put an upgrade on the server, there is still only one service running on the server side.
All our PC are set in the same way with restrictions to prevent parameters changes so why him ?

If anybody could give a tip, I'd appreciate :)

[solo]

https://www.vpnusers.com/viewtopic.php?f=7&t=69403 and its inner links. If no resolution, we'll focus on the single user next.