For some reason your plasma-desktop configuration became corrupted, perhaps as a result of the upgrade. The three commands respectively stop plasma-desktop, delete its configuration files and restart plasma-desktop. Logging out and back in also stops and restarts plasma-desktop. When plasma-desktop starts, if its configuration files do not exist they are re-created in their default format.
I'm not sure where you'd find documentation on this. You could start with the KDE web site: