This is not a question but a how-to as I figure that somebody else might encounter this bug as well.
Despite not updating anything or fiddling with any settings, all tray icons (not being plasma-applets) disappeared between a reboot. Icons like the network or audio volume worked just fine. Despite not showing tray icons (for example KeePassXC, Thunderbird, Telegram) those apps where still running but weren’t displayed or accessible in the System Tray.
After some searching I found the solution myself:
- Remove the package sni-qt including its settings with
sudo pacman -Rn sni-qt
- and reinstall that package with
sudo pacman -S sni-qt
- Reboot and the icons should have reappeard again.
The credit goes here: https://askubuntu.com/a/798653
Potentially relevant info:
- sni-qt 0.2.6-4
- plasma-desktop 5.10.1-1