I recently installed Kubuntu 18.04 (was using Xubuntu 16.04 earlier).
The overall impression is good but I was not satisfied with the functionality of some plasmoid widget that shows a menu of shortcuts to "Places".
So I tried to improve it and this led me to install several SDK and developpement packages/configuration for Plasma.
In the process I must have made a mistake because now the SystemTray (
org.kde.plasma.systemtray plasmoid) does not show the basic indicators like Network, Bluetooth, Clipboard, and so on. They do not appear in the configuration dialog either (where one can usually choose to hide them from the systray display). KDE Connect also does not appear, even though I can add an individual plasmoid to my panel for it that seems to be the same thing (so it is not broken per se).
However, the indicator for Redshift does appear ; it belongs to another category in the configuration dialog.
I understand from the Plasma SystemTray documentation that
kded5 is responsible for gathering the list of elements to be displayed, but the link about the details of that is dead.
I also think I understand that this is done via DBus but I could not find what for to search in