Revert killall -KILL Dock
So, fortunately I found where the problem was - it was the desktoppicture.db file located in ~/Library/Application Support/Dock/. Probably a corrupted one. Removing it made the macOS replaced it automagically with a new one :)
I would try ⌥⌘D first. If that doesn't work go to Apple Support Downloads, then search for, download, and run the combo 10.13.6 update (or the latest combo update available for your system version). That's may be the simplest, fastest way to fix something like this.