Ran into this issue yesterday. Updated MacOS with latest security patch (Mojave 10.14.6), opened a shared note from a coworker and it was 'locked' saying I was only granted read-only access. I verified via phone EN app and web interface that I still had full edit rights and after making an edit and then attempting to reopen on the Mac (v 7.13 - 458080 Direct) it has only displayed the "Note is unavailable. Make sure you are online" message. strangely enough when I open the note details it does show correct data about last update times for the note.
I have completely uninstalled and removed all preference and other folders to do a clean install of EN and the results are identical. This is a critical issue and massively impactful to my productivity.
Any suggestions as to how to get back to a working state are tremendously appreciated!