The short answer is that there is a device on your network at IP 169.254.8.76 which is responding to PMS’ SSDP broadcasts. These broadcasts are generally used to discover DLNA-capable players on the local network. Whatever is at that IP address is responding, so PMS is trying to query it for capabilities and such. If you’re not using DLNA on your network you can disable this functionality in Plex by unchecking the following in the PMS web UI:
Settings → DLNA → Enable the DLNA Server
You could also just ignore the error.
[Edited July 24th, 2020]
In case someone else stumbles across this post, and as it is being linked elsewhere, I’d like to clarify that the above is somewhat incorrect, or at least incomplete. While DNLA uses SSDP for device discovery, it is not the only Plex feature which does. So, disabling DLNA will not disable Plex’s use of SSDP discovery. It is used (at least) for a few other services, including network tuner support and creating automatic port forwarding using UPnP. Here’s a more recent post with some additional information:
And here’s a recently created feature suggestion for the ability to completely disable Plex’s use of SSDP:
