Comment 3 for bug 1821873

'Solved' the problem by adding a startup delay of 10 secs. in /etc/xdg/autostart/hplip-systray.desktop, replacing the existing 'Exec=' line like so:

Exec=bash -c "sleep 10 && hp-systray -x"

Apparently, my problem is a bug in the Gnome version of Ubuntu 20.10.