In short, on my work computer I have two different versions of Ubuntu installed, with roughly the same setup. I am connecting using H323 to a specific endpoint and I am getting different behavior.
On my older Ubuntu 16.04 distro, everything looks good, I (and others) can see the image from my webcam. On my newer Ubuntu 18.04 (Kubuntu actually), the image from my webcam seems to be completely black and this is confirmed by other participants, note that I can see their image fine. On this newer distro, the problem disappears if I deactivate h264 (and h263) and revert to h261 (although the image is at quite lower resolution. If I just test the webcam using the preview or try to see the local stream during a meeting it works fine.
I tried debugging the ekiga sessions using '-d 4', I was only able to spot some FFMPEG errors in the case of my newer distro ("FFMPEG non-existing PPS 0 referenced" and then "FFMPEG decode_slice_header error").