P: Videos don't play and don't import

did it, and now it's working ... many thanks for your support

My LR 7 is able to import and play that video file, so that rules out a problem with the file itself.  

You might try exiting LR and manually deleting the video media cache: https://forums.adobe.com/message/7553521#7553521

I have another issue: Tried to empty the video cache, does not work, LR is frozen, not possible to quit LR the normal way, killed LR with the task manager. After restart same issues as before.

same problem here

It's to all my video files ... and it worked in the version before Classic CC. Here is the link to a Dropbox folder with one of the video clips and the screenshot: 
https://www.dropbox.com/sh/6kgmdwiv43ljo8e/AACQYbo2P7N3yuVOeCuDx6S5a?dl=0

Importing video and then viewing it works in my LR 7. Some initial troubleshooting steps:

1. Can you please post a screenshot of the error message?

2. Upload a sample video to Dropbox or similar and post the sharing link here.  That will help identify whether the problem is specific to the video file(s) or to your particular installation of LR.

thanks for your reply, for the videos which were already in the old catalogue I see the thumbnails, but when I try to open it I get a grey window and after some seconds an error message like "working with the photo generates a failure" ( I have the German version "Beim Arbeiten mit dem Foto ist ein Fehler aufgetreten", I do not know the correct English wording).
Trying to import new videos into the catalog even does not work anymore.

In my LR Classic 7 catalog, video support is still present and functioning.  Initial troubleshooting:

1. In Library, do you see the video thumbnails?

2. What in detail happens when you double-click one of those thumbnails?