This comes occasionally. This is the result of Plex Media Server’s use of SSDP to discover other devices on your local network. While some functions can be disabled (GDM server discovery and DLNA) others cannot.
Plex is always going to send SSDP search requests on your local network and it will query devices which respond using the URL they provide (such as what you show above).
These errors can be safely ignored as they are not indicative of any “failure” of the server itself.
