(In reply to Michel Dänzer from comment #8)
> The Xorg log file indicates that Xorg starts up in 2560x1440, then an X
> client switches to 1920x1080 about 0.25s later (~23s after the kernel starts
> booting). Do you notice anything on the monitor around that time, e.g. some
> kind of flicker? What does the monitor say about the input signal when the
> black screen is shown?

After trying one more time I realized that the timing reported by the Xorg log
was really too high.
So I tried to leave the PC in this black screen for a couple of minutes and I
saw that the Xorg log entries were all starting at 140s+

Apparently it is not a static black screen but instead an endless loop? And the
Xorg log is only about the last boot attempt.

With a video it will be easier to show exactly what happens: 
https://youtu.be/YipyPIcP0lA

Moreover as you can see if I move the mouse I'm able to see the SteamOS cursor
for a fraction of a second before the monitor turns completely black again.

> Does amdgpu.dc=0 on the kernel command line make any difference?

No difference. I'll upload the Xorg log and the dmesg of it anyway, just in
case it could be of any help.