And updated Kubuntu, and after a reboot a few hours later, I got the usual BIOS/UEFI display and then a black screen (instead of the usual X session display). The session processes (Firefox…) started anyway (could see them using a ssh session from my phone). TLDR: it turned out that is was probably the HDMI cable that was not properly plugged. Crazy.

Same issue with the previous kernel version. Also, when I started Kubuntu in « recovery » mode, the console displayed properly. And when I started the OS with « nomodeset » in the kernel parameters (without graphic card acceleration), then I got a working X session in a low resolution.

So I looked in the logs with journalctl, and I could see that I had these lines:

# before, when working
[drm] Initialized amdgpu 3.57.0 20150101 for 0000:29:00.0 on minor 1
fbcon: amdgpudrmfb (fb0) is primary device
amdgpu 0000:29:00.0: [drm] fb0: amdgpudrmfb frame buffer device

# after, with black screen
[drm] Initialized amdgpu 3.57.0 20150101 for 0000:29:00.0 on minor 1
amdgpu 0000:29:00.0: [drm] Cannot find any crtc or sizes

Maybe some Ubuntu package was updated and prevented the AMD driver to be loaded? Or maybe some X configuration file was corrupted. Then I found this messages thread, stating:

HDMI connections are very sensitive to small misalignments. This type of issue shows up quite a bit in forum posts

So I unplugged and re-plugged the HDMI cable (which was not screwed), rebooted, and it started working properly. I would have thought that since it’s digital it would either work properly or not at all.