I have never been able to connect to my Plex Media Server securely on my local area network so I decided to fix it today. The official troubleshooting article mentions pfSense 2.2 which I run. It tells users to configure DNS rebinding. This was not enough in my case, so I searched some more for a solution. The forums had a hint about how to solve this in a post by snm77. His suggestion is to add a host override point plex.direct to your servers local IP address. This did not work, but pointing the FQDN did! The FQDN can be found by using the inspector in your browser and reload https://app.plex.tv/web/app , in Chrome you would look at the Network tab and find connections to IP.*.plex.direct, the one with your servers IP is your FQDN.
In pfSense 2.2+ do the following:
1) Configure DNS Rebinding by going to System > Advanced > Admin Access and enter plex.direct under Alternate Hostnames.
2) Configure Host Override by going to Services > DNS Forwarder and create a new entry under Host Overrides. Enter the IP part for your FQDN in the Host field, the rest in the Domain field and enter your servers local IP in the IP address field. Like the image below:
After these configurations, reload with the new DNS settings and you will have secure connections on both the internal network and on the internet!