I have this issue too. It seems to happen when I create a new Note, and click the "Go to Notebook" icon at the top. I click on that on accident, as i believe clicking it used to allow you to change the notebook. After clicking it, the note becomes uneditable, and.I have to restart.
I'm experiencing this on OSX Catalina with Evernote 10.8.5-mac-ddl-public (2367)