I set up a new ubuntu 17.10 and installed dolphin from Ubuntu software. When I do a search ('find'), after typing a few characters a message shows up 'invalid protocol'. Any packages missing to be installed?
You could try the instructions here: https://www.ryananddebi.com/2015/07/13/linux-getting-find-working-in-dolphin-on-kde-linux-mint-and-kubuntu/
In short install kde-baseapps, systemsettings and kfind. And DISABLE "Enable Desktop Search" in Systemsettings -> Desktop Search. It should work then. It did for me. Pretty counterintuitive but it works ;).