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
This affects my setup and is (I think) the only blocker keeping me from using the open source drivers (which I would prefer). Right now I need to use fglrx to support my 6 monitor setup, 4 of which are daisy-chained on DisplayPort with the last member of the chain using a DisplayPort to DVI adapter.
If there are any proposed fixes, I would be happy to try them out.
I have tried to run the newest kernel 4.3.0 (http://ubuntuhandbook.org/index.php/2015/11/linux-kernel-4-3-released/) and got exactly the same issues.
I have tried with arch kernel 4.4.1, also same issue
I believe I am seeing the same issue. I posted a thread on the Arch Linux forums for guidance, it contains all the details, it is here: https://bbs.archlinux.org/viewtopic.php?id=210338
I am using Linux 4.2.5-1
It still doesn't work on Ubuntu 17.10 (4.13.0-16-lowlatency).