VPN L2TP not working since last update

On a USG20-VPN, I set up a VPN-L2TP working perfectly fine for several users.
Since several weeks (it seems to be since last update) no one is able to connect on the VPN getting this error on Windows (sorry in French):

On the USG20-VPN, I see only logs with IKE filter:

Thanks for your help...

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