Probably a common issue, but I could not find a solution yet. I’m on OpenSUSE Leap 15.5. Using Dolphin. Even though I have the thumbnail option activated for videos, I don’t get the thumbnail for working.
Turned out that I already have ffmpegthumbs installed. What I did was to uninstall ffmpegthumbnailer and libffmpegthumbnailer4. These are the current ffmpeg packages I have installed.