Help me solve several problems with Redshift on Lubuntu 21.04

I had the same problem after upgrading from 20.10 to 21.04.

It seems that there is one instance started by systemd (which crashes and gets restarted several times) and one instance started via the app itself by enabling autostart (which creates an entry ~/.config/autostart/redshift-gtk.desktop)

By removing redshift-gtk.desktop the flickering stopped, but redshift-gtk still crashed. I assume that it was either started to early or was missing a DISPLAY env-variable.

I had a hard time to remove redshift from systemd. Disabling or masking the redshift-gtk.service did not work. Still crashed on startup. I found that you have to disable the service with:

sudo systemctl --global disable redshift-gtk
sudo systemctl --global disable redshift

If you then enable the autostart via ~/.config/autostart/redshift-gtk.desktop it works as expected.