K900
Install `kdepim-addons`.
How did you install it?
Uhh, excuse me? Why are you reporting issues from Kubuntu on a NixOS bug tracker?
Which Akonadi version is this on?
Can someone see if this reproduces on 25.08 and 6.4.4?
No, Plasma 6.4.4 is what you want.
OK, can you post the contents of your `~/.config/plasma-org.kde.plasma.desktop-appletsrc`?