I got my note 20 (not ultra) a few days ago and as I use snapchat, the snap camera turns black with message to clear app or restart phone. Its happened twice now.. Then I go to see if the stock phone camera is working but it has a black, screen also. Then I try facebook messenger camera and it is also black with a message. I came from s10 plus and this never happened. If anyone knows if its just the app app causing problems or if its the phone. Doesn't work until phone is restarted.
go to Settings, and search for permission manager. click on it. then scroll down till you see Camera. click it. every app you gave permissions of the camera to, will be listed. look through for any 3RD PARTY APP YOU DOWNLOAD that has access to camera, even if its not a camera app, and just start hitting DENY one by one and see if the camera works. if it doesn't enable again and go to the next one and try until you find the culprit