I do see a remote access functional only for a period of time. The Fritz!Box Router (7490, actual Firmware) is flooded with UPnP Port Forwarding requests from the Plex Server (all the same). It continues to be functional as soon as I manually delete all the UPnP opened port forwarding. It seems that there is no cleanup happening.
When switching to manual port forwarding the remote access is green for half a second.
Plex Server on macOS 10.12
Static IP Adress via IP-Adress Reservation
IP Lease Time 2 days (reduced from 10 days for testing)
Actual Plex Pass Version 1.2.3.2914