I was running a fairly old version of the Events Calendar. There was a note about a bug in versions below v69, which was triggered by the recent Qt 5.15.1 update:
Note: v69 is required for Arch/OpenSuse! v68 and below crash in Arch after the Qt 5.15.1 update.
Black Screen on Login / Crash on Arch/Manjaro (Qt 5.15.1) (EventCalendar v68 and below) · Issue #154 · Zren/plasma-applet-eventcalendar · GitHub
I used the commands from the link below to know that I was running a much older version of Event Calendar (v63), and that it was still in place, even though I thought it had been uninstalled:
grep Version ~/.local/share/plasma/plasmoids/org.kde.plasma.eventcalendar/metadata.desktop /usr/share/plasma/plasmoids/org.kde.plasma.eventcalendar/metadata.desktop
So I removed the following folder:
I was then able to reinstall Event Calendar v69. It seems to be working fine again and Latte Dock isn’t crashing any more.
I installed using AUR instead of using the Plasma Widget installer, this in the hopes it will stay updated: