Hosting a VPN with Airport Extreme
First check the settings on the AirPort Extreme to ensure that the box labelled "Allow incoming IPSec authentication" in "Network Options..." under "Network" is checked.

Then I suspect what have happened is that your Mac Mini has received a different internal IP address. This could lead to the port forwardings that you have had setup not working anymore, which is confirmed by the fact that you cannot see your port forwarding of TCP port 1723 with a port scanner.

On the Mac Mini open System Preferences and Network, and find the internal IP address of your Mac Mini. It might be for example 192.168.2.10. Then check the port forwardings on the Airport Extreme under "Network" and "Port Settings...". Ensure that the field "Private IP Address" lists the internal IP address of your Mac Mini for all the VPN related ports  (UDP ports 500, 1701, 4500 and TCP port 1723).