Dolphin does not show thumbnail-images of video files, despite the required thumbnailers have been installed and the respective settings within dolphin have been made. 

The following line of code fixes the problem: 
sudo ln -s /usr/lib/x86_64-linux-gnu/plugins/* /usr/lib/x86_64-linux-gnu/qt5/plugins/ 

Reproducible: Always

Steps to Reproduce:
1. install ffmpegthumbs, ffmpegthumbnailer, kffmpegthumbnailer, mplayerthumbs
2. Go to Dolphin Settings, activate Preview for Video files
3. Open a Folder that has video files 

Actual Results:  
Only Symbols are shown for videos, but no actual preview

Expected Results:  
Show some thumbnail image of the video

The following line of code fixes the problem: 
sudo ln -s /usr/lib/x86_64-linux-gnu/plugins/* /usr/lib/x86_64-linux-gnu/qt5/plugins/ 

Other Versions of Dolphin could also be affected, because different Versions of the Plasma environment seem to be afffected.