Hi,
I have exactly the same issue with my Samsung A7 since yesterday. Half of the notes I am editing are blocked as soon as I am saving them. And they can't be deleted (I can put it in the recycle bin, but that's all)
The only solution I found so far is to copy the content of a note manually with copy and paste into a new note and then desintall the app. When I install it again, the "locked" notes have disapeared