Now had the same problem on another Devuan system, Daedalus release, where both packages (xfce4-power-manager, x11-session-utils) were installed and reinstalling didn’t help, but removing display.xml from $HOME/.config/xfce4/xfconf/xfce-perchannel-xml/ did (as per wiki archlinux Xfce: Black_screens_at_boot_with_NVIDIA_and_multiple_monitors).
@ghp Tried that too, doesn’t work. Thanks anyway - keep them coming… there must be something that works 