I installed an older kernel, and was able to boot and log in. But the external display (HDMI) still didn't work. In desperation I went to Settings -> Users -> Automatic Login.
And now everything works. Apart from the fact that my computer can be used by anyone. So it was a mistake on my part to tag this as a xorg problem. It's clearly to do with the login of the window manager.
I installed an older kernel, and was able to boot and log in. But the external display (HDMI) still didn't work. In desperation I went to Settings -> Users -> Automatic Login.
And now everything works. Apart from the fact that my computer can be used by anyone. So it was a mistake on my part to tag this as a xorg problem. It's clearly to do with the login of the window manager.