Import and Playback of videos no longer works

After switching to Lightroom Classic12, videos (e.g. mp4 from Google Pixel or Canon R6) can no longer be imported.
Also previously imported mp4 files are no longer played back by LR.
Am I alone with this problem or are other users also affected? How can the error be eliminated?

The hints on https://helpx.adobe.com/de/lightroom-classic/kb/troubleshoot-video-issues.html unfortunately did not bring a solution.