I have this problem every day, but it doesn't happen on opening the application. At seemingly random points in using the app (I have about a dozen notebooks), clicking from one notebook to another causes the content pane to be empty. (When it happens, I have usually opened a note via Search, and, when done with it, clicked on a different notebook.) The only way I've found to get content back is to close and re-open the app. I'm on a Mac running OS 11.0.1 and 10.5.7 of Evernote. Also, I get spurious popups showing me that I am running the latest version of Evernote as though I had asked it to try to update.