I’m also seeing this problem on v1.14.1.5488-cc260c476 running in Docker with host network, and using an OpenWrt router running miniupnpd 2.1.20180706-1. As I’m not willing to disable UPnP on my router, I decided to find a workaround.
At first I tried to add a firewall rule on the Docker host to drop packets with destination port 1900. This didn’t help however. After some tcpdumping, I noticed that Plex contacts the UPnP daemon, and receives a reply on a port other than 1900, but with source port 1900, so I added a firewall rule to drop packets with source port 1900:
iptables -I INPUT -p udp --sport 1900 -j DROP
Then restarted the Docker container, and the problem hasn’t appeared since. Hope this helps someone until the problem is resolved in Plex or miniupnpd.
