Hi. Sorry for the late reply. As you have understood, for video thumbnails to work, FFmpeg must be installed and available from Linux command-line, and exec() function must be available from PHP.
atallen I can enter ffmpeg -version at Strato via SSH. As however, the response is "Command not found".
The same result also in the correct folder.
Ok, so this would mean that FFmpeg is not installed, or maybe it's installed in a specific location where you need to point the path. I think the common way to install on Linux would be from command-line:
sudo apt install ffmpeg
Please try the above. It's the recommended way to install if you search "Install FFmpeg on Linxux". After installing, try to run ffmpeg -version to check if it's installed.