The properties on the udpsink/udpsrc elements need to be set before there is any state change. If not, in a network without default gateway, udpsink tries to bind an a NULL interface and fails. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1411>
See commit 921e9a54: rtpsink: set sync off on rtcp_sink Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/993>