This error message is shown every time I log in. It is shown twice in quick succession. The exact content (in Finnish) is as follows, translation below.

Noutotyön virhe
Plasma työpöytäympäiristö
Tuntematon virhe - Unable to fetch item from backend.

(Fetch job error
Plasma desktop environment
Unknown error)

I suspect that this error is related to the settings for Gamma in the Display section of the plasma settings, because that is the last thing I changed, and the settings are not applied when I log in. I always have to open the Settings dialog and go to Gamma, at that moment Plasma remembers that Gamma settings, shows me the correct slider positions, and the display adjusts. (Just by opening the Gamma page.) Maybe the "item" that fails for fetching has to do with this.

Additional information: In the Gamma settings dialog, there is a box for "system-wide" which asks for the root password. I've tried setting gamma settings both with that option checked and with it unchecked, but they are not applied automatically at login to KDE, no matter how I set them.