Dolphin's "Show previews for" blank
Solution 1:
ffmpegthumbs 4:15.12.3-0ubuntu1 is buggy and unpacks ffmpegthumbs.so to the wrong location. Until the package is fixed a workaround is to link the files to the correct location:
sudo ln -s /usr/lib/x86_64-linux-gnu/plugins/* /usr/lib/x86_64-linux-gnu/qt5/plugins/
This works on my installation of Kubuntu 16.04 LTS. By the way ffmpegthumbs needs kio-extras to work (should be installed on Kubuntu by default as a dependency of kubuntu-desktop).