Now it's behaving differently. Only some notes cannot be edited. (Both web and desktop). It looks like it has effected only my most recently edited notes. In one such note, the information appears in the preview pane, but if you choose "open note" from the context menu, the note opens with no text at all, not even the text that appears in the preview.
In other notes, you cannot get an active cursor to appear within the note, no matter how you click on the note or request to "edit" it from the drop-down menu. It's like you're clicking on nothing at all and the note can't be edited. In those notes, however, you can choose "open note" from the context menu and the cursor will appear within the opened note. You can then move the cursor around with the arrow keys and edit with it. However, if you ever use the mouse to click anywhere in the note itself, the cursor disappears and you've been "kicked out" of the note. The open note is still in front of you but you can no longer edit it.
It has something to do with mouse commands. The program no longer likes it when I use my mouse to try to edit some notes.
I've switched over to One Note so I can have something that works.