After one of the latest updates, for me it was on Friday 13.03.2020, I noticed black backgrounds under my polybar and conky. I figured something was wrong with compositor transparency.
For me, compositor is being started from '
~./config/openbox/autostart' by '
That script was changed with the latest update to use picom instead of compton and requires '
$HOME/.config/picom/picom.conf' to exist.
For me that folder and conf file was missing so compositor wasn't starting at boot.
I fixed it by renaming my old compton configuration file and adding it to the new directory.
mkdir ~/.config/picom cp ~/.config/compton.conf ~/.config/picom mv ~/.config/picom/compton.conf ~/.config/picom/picom.conf
Picom started normally after a reboot.