You can safely ignore these log entries. They’re the result of Plex Media Server sending SSDP multicast queries on your network. Without getting too deep in the weeds, when your Fire TV replies it does so with a URL from which the querying server (Plex) can ask for more information. When Plex accesses this URL, it doesn’t find the schema it expects and logs the error.
There is currently no way (in Plex) to disable these queries entirely. The best you can do is use a firewall on your server to block certain traffic to UDP port 1900 (or block SSDP replies from the offending devices).
Another recent thread:
