When updating KDE in my Fedora 23, I lost the video previews in Dolphin. Even after installing ffmpegthumbs did not work.
So here is the solution:
sudo cp /usr/share/kde4/services/ffmpegthumbs.desktop /usr/share/kservices5/ sudo cp /usr/lib64/kde4/ffmpegthumbs.so /usr/lib64/qt5/plugins/
You need to have installed ffmpegthumbs for this to work, to install it:
sudo dnf install ffmpegthumbs
And you need to enable the video preview again in Dolphin.