Q: OS X Server - Windows client

Hi, since a few days I have a OS X Server running on my Mac Mini. Everything works like a charm. Except the VPN connection.

My iOS devices (iPhone 6 and a iPad Air) connect without a problem. Now for the last part I am trying to connect my Windows 10 laptop to my VPN. And when I'm connecting I am getting this error:

I used the steps below:

• Click "PPP Settings..." under the Options tab, check "Negotiate multi-link for single-link connections".
• Under Security Tab, set Type of VPN to L2TP/IPsec.
• Click Advanced settings, enter/paste your pre-shared key.
• Then follow the steps in Apple's Support page herehttps://support.apple.com/en-us/HT202384.
• Restart and you should connect.

Any tips for me?