I have 2 monitors connected to an AMD R9 380 GPU, one with DisplayPort (Primary) and one with HDMI (Secondary).
I first tried to use the secondary a few days ago without any problems, however when I tried to activate it a second time today the primary flashes off and back on (as normally happens when a second display is added) and the Keep/Revert dialog is shown, however the secondary remains off.
Looking through the syslog, I believe this error is the culprit as it occurs on each occasion I try to activate the secondary:
Config not applicable: GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: The requested configuration is based on stale information
I found this related issue and tried the suggestions noted in the top 2 answers with no success.
I'm using Ubuntu 18.04 with all updates installed and no additional drivers or kernel modules.