Hi @TOM-RFB, let me see if I can help out here.
I have translated your question to English as this is an International forum, please post any replies in English and it will be easier for me and other users to assist you.
So if I understand you correctly, the camera app works fine for photos but when you switch to video capture, you receive an error message saying that the camera is used by another app? Have you tried clearing the data for the camera app? Tis will not erase any previously taken photos or videos, but it will reset all settings for the camera app which might do the trick. Find and tap Settings > Apps & notifications > Show all XX apps > Camera > Storage > Clear data > Restart the phone.
Another thing that you can try is to see if you are able to capture a video while in safe mode. Safe mode is a troubleshooting mode where all apps downloaded from Play Store is temporarily deactivated, that way you can see whether this behaviour is caused by a downloaded app or not. For more information on safe mode and how to use it, click here.
Try this and let me know how it goes, and please return if I have misinterpreted your question in any way!
