gscreamrx / RTCP feedback problem ?

Hello!
i'm assuming this is possibly a dumb "user error" issue...
but i'm having trouble figuring out what's happening here. I have a video that i'm trying to stream from one ubuntu18.04 box to another.. and was seeing lots of "RTP queue discarded for SSRC" messages showing up at the sender. I take it this means RTCP feedback is received, but something like either IP addresses are not properly set, or ports are blocked (as i read in another thread here).
Of course the video is very laggy and poor quality. I can run these pipes without gscreamtx/rx and it works well.

(to make testing simpler... i actually ran the two pipes colocated on a single machine as you can see below):

Here's my sender:
gst-launch-1.0 rtpbin name=rtpbin ! filesrc location=tears_of_steel_1080p.mov ! decodebin ! x264enc name=video tune=zerolatency ! rtph264pay ! gscreamtx media-src=0 ! rtpbin.send_rtp_sink_0 rtpbin.send_rtp_src_0 ! udpsink host=135.112.62.140 port=5000 rtpbin.send_rtcp_src_0 ! udpsink host=127.0.0.1 port=5001 udpsrc port=5001 ! rtpbin.recv_rtcp_sink_0

Here's my receiver:
gst-launch-1.0 rtpbin name=rtpbin udpsrc port=5000 ! gscreamrx ! application/x-rtp,media=video,clock-rate=90000,encoding-name=H264 ! rtpbin.recv_rtp_sink_0 rtpbin. ! rtph264depay ! avdec_h264 ! videoconvert ! xvimagesink sync=false async=false udpsrc port=5001 ! rtpbin.recv_rtcp_sink_0 rtpbin.send_rtcp_src_0 ! udpsink port=5001 host=127.0.0.1 sync=false async=false

the receiver on start shows this error:
```
*************INIT*************

Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...

(gst-launch-1.0:5129): GLib-GObject-CRITICAL **: 11:09:07.354: g_object_set: assertion 'G_IS_OBJECT (object)' failed
SINK EVENT

ENABLE CALLBACK

Setting pipeline to PLAYING ...
```

once i set GST_DEBUG=4, i can see this on startup:

```
0:00:00.414568983 5470 0x55d3473aa190 INFO GST_ELEMENT_PADS gstelement.c:917:gst_element_get_static_pad: no such pad 'sink' in element "udpsrc0"
Pipeline is live and does not need PREROLL ...
0:00:00.414617433 5470 0x55d3473aa190 INFO GST_PARENTAGE gstbin.c:4466:gst_bin_get_by_name: [pipeline0]: looking up child element rtpbin

(gst-launch-1.0:5470): GLib-GObject-CRITICAL **: 11:14:20.982: g_object_set: assertion 'G_IS_OBJECT (object)' failed
Setting pipeline to PLAYING ...
0:00:00.415221686 5470 0x55d347051200 INFO GST_EVENT gstevent.c:1388:gst_event_new_latency: creating latency event 0:00:00.000000000
0:00:00.415279838 5470 0x55d347051200 INFO bin gstbin.c:2781:gst_bin_do_latency_func: configured latency of 0:00:00.000000000

```
Any ideas here?
This is a problem with udpsrc setup?" for the rtpbin?

thanks so much!
-pete