Server app. How to fix / re-enable broken VPN after update to Mojave?
These instructions are largely the same as the ones in the official PDF-file that goes through the migration progress, but with the (hopefully) helpful addition that the the incorrect command is corrected and the other commands are copy:able as-is.


Turn off the VPN service in the Server app. (If visible. For me it was not.)
Make sure you are in the wheel group. I followed these steps (link )
Open Terminal (where the rest of the instructions will be executed)  and go to the appropriate directory with:

cd /Library/LaunchDaemons
Create an empty file called vpn.ppp.l2tp.plist in current directory with: 

sudo touch vpn.ppp.l2tp.plist
Set the right owner for the file with

sudo chown root:wheel ./vpn.ppp.l2tp.plist
Open the file with nano (to avoid problems with saving that can occur if you use TextEdit or similar):

sudo nano vpn.ppp.l2tp.plist
Copy paste the content below into nano:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
“http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
  &lt;dict&gt;
    &lt;key&gt;Disabled&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;EnableTransactions&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;vpn.ppp.l2tp&lt;/string&gt;
    &lt;key&gt;KeepAlive&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;Program&lt;/key&gt;
    &lt;string&gt;/usr/sbin/vpnd&lt;/string&gt;
    &lt;key&gt;ProgramArguments&lt;/key&gt;
    &lt;array&gt;
      &lt;string&gt;vpnd&lt;/string&gt;
      &lt;string&gt;-x&lt;/string&gt;
      &lt;string&gt;-i&lt;/string&gt;
      &lt;string&gt;com.apple.ppp.l2tp&lt;/string&gt;
    &lt;/array&gt;
    &lt;key&gt;EnableTransactions&lt;/key&gt;
    &lt;false/&gt;
    &lt;key&gt;EnablePressuredExit&lt;/key&gt;
    &lt;false/&gt;
  &lt;/dict&gt;
&lt;/plist&gt;

Save with changes by pressing Control + o, confirm file name with Enter and close nano with Control + q.
Load your new file by running:

sudo launchctl load -w ./vpn.ppp.l2tp.plist
Verify with:

launchctl print system/vpn.ppp.l2tp


These steps worked for me. Hopefully they can help for someone else that, like me, struggled to get the VPN feature up and running again after macOS Mojave update.

The official PDF guide (mentioned above) has some additional information about "Ongoing management":


  Settings can be changed after vpnd is configured by editing the
  /Library/Preferences/SystemConfiguration/
  com.apple.RemoteAccessServers.plist file. Refer to the vpnd (5) man page for details on the configuration format. After changes are made, you can have the service reread the configuration file by executing the command
  sudo killall -HUP vpnd.