Downgrading all these packages to their previous version in Debian Testing (4.14.2-1) resulted in a working Dolphin again: dolphin kde-baseapps-bin kde-baseapps-data kdebase-bin kdepasswd kfind konqueror konqueror-nsplugins libkcddb4 libkonq-common libkonq5-dev libkonq5-temp libkonq5abi1 libkonqsidebarplugin4a plasma-widget-folderview