Some weeks or months back I used to be able to have a secondary monitor
with XFCE on Ascii. Lately when I try attaching a second monitor,
although Settings Manager -> Display can find the monitor, no signal
gets sent when I set the display manager to use it. Same for xrandr, it
can see the second monitor but apparently not send a signal to it. The
monitor just says "HDMI-1 No Signal" rather than showing XFCE.

The same monitor + cable works fine with other systems.

Some weeks or months ago, it was just a matter of plugging in the cable
and it worked fine in Ascii though not in Jessie. What do I need to
change or reconfigure to regain use of a second monitor in Ascii with XFCE?

Regards,
Lars

$ lsb_release -rd
Description:    Devuan GNU/Linux 2.0 (ascii)
Release:        2.0

$ uname -sr
Linux 4.9.0-3-amd64

$ xrandr --listmonitors
Monitors: 2
0: +*eDP-1 1920/290x1080/170+0+0 eDP-1
1: +HDMI-1 1920/531x1080/299+1920+0 HDMI-1

$ xrandr --output eDP-1 --auto --output HDMI-1 --auto --right-of eDP-1