Comment 27 for bug 1861609

Thank you, Daniel. I went ahead and posted my findings over there too.

One more thing I wanted to add over here: I've noticed a few people say that uninstalling libva-wayland2 also fixes it. I can confirm that too, but I looked deeper and discovered that what's actually fixing it is uninstalling gstreamer1.0-vaapi. When you uninstall libva-wayland2, apt also uninstalls gstreamer1.0-vaapi.

If you leave libva-wayland2 installed and uninstall gstreamer1.0-vaapi instead, that also fixes the crash. In particular, the file /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstvaapi.so seems to be the culprit (that's the main file installed by gstreamer1.0-vaapi). If I just move that file to my desktop and reboot, the Xorg crashing problem goes away.

I'm not sure about the interaction between that GStreamer plugin and DRI2Authenticate being called too early, but it's definitely an interesting clue. At first I thought this was nonsense -- how could a GStreamer plugin cause Xorg to crash on login? -- but sure enough, it seems to be involved at some level.