There are many, many threads dealing with this issue that you might have found if you’d searched first.
It’s most likely an interaction with UPnP on your router if you’re running OpenWrt or some other system that utilises minupnpd. The problem has been identified by Plex staff, test builds have been created that resolve the issue and we’re waiting for it to go into a production build.
Much more information here:
Quick fix is to disable UPnP on your router and restart PMS, but be aware of the impact this may have on other devices on your network.
