Submitted by Adrian
Assigned to Default DRI bug account
Details have been been submitted here: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/1509997
Today (3.11.2015) I have tried with the newest version from ppa:xorg-edgers/ppa and even with apparently the newest version of the Radeon driver, the problem still exists (the same symptoms) and all screens switch to the standby mode.
The only error message I have managed to spot is:
[drm:radeon_mst_encoder_dpms [radeon]] ERROR DPMS OFF 0