Regarding the first issue, it is likely related to the health check being performed by Docker. This thread has some information (there are likely others, but this is the first one I found):
Regarding the second issue, it is the result of Plex Media Server sending SSDP discovery requests on the local network. You have a device (or devices) on your network which is (are) responding, but when queried further replies with invalid information. Plex provides no means by which you can fully disable its usage of SSDP discovery as it is integral to several features. The error messages themselves can be safely ignored, however the errors will always be logged.
You can use iptables or UFW to block outgoing messages to UDP port 1900 (or responses from the specific device) if it causes issues, but be aware it will break certain things (like network tuner automatic discovery).
